Sudden Aggression

 Find a different place to run the dog.  Dog parks are not that great a place for all dogs.  Too much stimulation and you can not control the other dogs.

Unfortunately I'd recommend not taking him there.  Many if not most dogs are not dog park material.  If your dog is just turning two that makes perfect sense.  That's about the time a dog is mature mentally and often times their behavior changes, not necessarily for the worse but you start to see a dog's true colors.  For many dogs, especially mature male dogs, having a large pack of all different dogs in a small area just isn't fun anymore.  If there are a few dogs your dog gets along with, I'd suggest play dates instead.  Who is starting fights and why isn't really important here. If fights are happening and your dog is involved, it's not a good environment for the dog whether he started them or not.  To me what you describe doesn't really sound like fights (the first thing sounds like your dog telling the other dogs off for chasing him and the other thing along the fence sounds like your dog getting over stimulated by fence running and redirecting at another dog) but if he's starting to get snarky there is definitely potential for escalation and real fights.

Liesje

Unfortunately I'd recommend not taking him there.  Many if not most dogs are not dog park material.  If your dog is just turning two that makes perfect sense.  That's about the time a dog is mature mentally and often times their behavior changes, not necessarily for the worse but you start to see a dog's true colors.  For many dogs, especially mature male dogs, having a large pack of all different dogs in a small area just isn't fun anymore.  If there are a few dogs your dog gets along with, I'd suggest play dates instead.  Who is starting fights and why isn't really important here. If fights are happening and your dog is involved, it's not a good environment for the dog whether he started them or not.  To me what you describe doesn't really sound like fights (the first thing sounds like your dog telling the other dogs off for chasing him and the other thing along the fence sounds like your dog getting over stimulated by fence running and redirecting at another dog) but if he's starting to get snarky there is definitely potential for escalation and real fights.

What she said.

Keep in mind that you could be facing some major problems if your dog attacks and injures another dog or if there is a real dog fight and another dog's owner is injured trying to break it up.  If you just can't figure out a way to exercise your dog without going to a dog park, go when it's least crowded and hope for the best.  Most dog behavior experts are opposed to dog parks for multiple reasons.  Another consideration is how your dog attacking another dog might have long lasting effects on the other dog.  Just not fair to the other people at the dog park, IMO.  

The behavior you describe is normal so I can't suggest any training methods that would help you. You may be setting your dog up to become truly dog aggressive and that could carry over into all dog encounters, not just at the dog park.  Not all dogs are cut out to be dog park dogs.

I'm not referring to the OP personally but many people use dog parks as the lazy way to exercise their dogs.  I get that but there are other ways to get your dog exercise.  They require an owner willing to do a more than just turn their dog loose in the dog park.

I take my dogs to a fenced in ballpark, where we have persmission to let them run and play. 

Ugh, I have the same problem.....it bums me out that I can't take my boys to the park

I know where you are coming from, my dog was perfect at the dog park until about 15 months old, then she started seeming to definitely tell other dogs to back off when they came after a toy she had found.  The worst time a much larger dog jumped her and pushed her to the ground for a tennis ball and began biting my dog's neck.  I have not taken her back to that overcrowded park again.

 My dog is about 30 lbs, so she does ok in the one park in town that has a side for small dogs only.  I can still take her there if there are not an overwhelming number of dogs, but I've found large numbers of big dogs put her on the defensive and she feels the need to tell others to give her some space sometimes, and some other dogs don't like that at all.

I have a good friend with a dog close in age to mine (she is 17 months) and they have been buddies since they were pups.  He is 3 times her size but they play wonderfully together so we get them together for play dates.  I take her to the small dog park and have taken her to a regular park during an off time when I was on vacation, there weren't many dogs there at all then and she did great.  I go prepared to leave even then if I don't like something I see in the least.  We also have a place in town with a lot of land owned by a local horse race track that doesn't mind if folks let their dogs run around a bit off lead.  We take her to doggie daycare for a day every couple of weeks at least also.  She absolutely loves it there.

I wish you luck.  I'm fortunate in that my dog loves to chase a tennis ball and she is my walking/running buddy in the morning, so she still gets exercise, but socialization is important.  I just want it to be good socialization, and a crowded dog park with unknown very large dogs that put her on the defensive isn't doing her any favors in socialization at all.

I'm going to address your post one item at a time. 

My dog has recently started acting aggressive and dominant towards other dogs. Most aggressive behavior stems from fear, so please don't assume that dominance is the cause. 

He is just about 2 years old and has been going to the dog park several times a week since he was vaccinated and could do so.  Puppies that wait until after they have been completely vaccinated before they begin socializing often miss valuable social lessons from other puppies.  The American Veterinary Society has info on this here:  http://www.avsabonline.org/avsabonline/images/stories/Position_Statements/puppy%20socialization.pdf.  I encourage owners to get their puppies to class no later than 12 weeks of age.  When a dog hits age 2, earlier for some later for others, they are at the age of social maturity and may be ready to tell other dogs that they're tired of taking crap from them. 

He loves to be chased and has always acted very submissive when other dogs have caught him or surrounded him. It usually ends up with him on his back. It's possible that he *doesn't *enjoy this as much as you think, or that he is overwhelmed by it, or that he is now mature and doesn't want to do it any more - some dogs don't play for a lifetime.  Herding breeds often tire of dog play by the time they are mature.  Labs tend to play for a lifetime.  Even then, individual dogs of all breeds may surprise us one way or another.  Watch his body language - all of it, not just the rolling on his back.  Is he always looking back, or stopping and dodging and chasing other dogs back?  Is he always the one being chased?  One sided chasing may be a sign that he is a target for some of the other dogs' bullying.

The only time he has ever acted aggressively is when there is a frisbee involved and if a dog tries to take it from him he has snapped before.  Not unusual.  Dogs often protect items from other dogs.

He has never snapped at a dog prior to this past week for any other reason. These past couple of weeks leading up to the incident he would start to jump on dogs backs and hump them if they wouldn't chase them. I noticed this was mostly with dogs younger than him usually around 1 year old or so. This week he was getting chased by a couple of pit bulls which he usually loves and then they stop and one of them basically body checks him and then he just started snapping at it very aggressively (definitely wasn't playful).   What makes you think this is OK - if I were him, I would have been ticked off, too.  By the way, owners who let their Pits exhibit this body slamming behavior are asking for trouble.  It's bullying, and evidence that their dogs were never properly corrected for it by an adult dog when they were little.  That is not a park I would take my own dog to, and I have a hound who loves to play.   If you go to my web site on the Yappy Hour page, you'll see him playing normally with a Boston Terrier.  Normal play should never look like there's no give and take, or that one dog always takes the brunt of whatever game... http://www.pawsforpraise.com/Yappy_Hour.html  Scroll down to the link at the bottom.

A different pit bull but from the same owner was running along side the small dog park basically chasing the small ones up and down the fence and my dog started running along side him and then another small fight broke up. I couldn't tell if he started this one but the owner asked me "if my dog had a problem because her dogs never fight". Many Pit owners are quick to defend their dogs (can you blame them?) but fence running, and chasing smaller dogs can lead to "predatory drift" in any terrier, or indeed in any dog with those tendencies, and the first thing that a dog in that mode does when interrupted is often to redirect the frenzy on to the nearest dog (or person, which is why people often get bitten in similar circumstances - not because the dog is after them, but because they interrupted a predatory sequence of behavior).

He never shows any aggressive behavior on his leash or in the house towards other dogs and at the park it's difficult to reprimand him when he's off the leash.  If you are reprimanding him when he is just trying to survive, or defend himself or his possessions, you may be accidentally contributing to the problem.  There are many unintended consequences to punishment, and until people understand them all, it's best to stick to cues.  If your dog will not "come" immediately when you call, maybe he doesn't belong at the dog park until that skill is cemented.

Does anyone have any recommendations? He has so much energy that I couldn't imagine not taking him there.  Not all dogs are dog park dogs, and yet they still need exercise.  I think your dog is in a situation that will only lead to further aggression.  If you have noticed him really liking a few of the dogs, maybe you could ask those dogs' owners to form a smaller play group in someone's fenced yard or early am in a local tennis court.  Or, you can do what I do with my one dog who isn't that fond of large groups.  I throw the frisbee till my arm falls off lol.  There's a machine called Go Dog Go that you can teach your dog to use - it's a tennis ball launcher.  You can also go to classes and learn to do a dog sport, such as agility, tricks, rally, etc.  Mental exercise is much more tiring than all that running around, believe it or not.  Get a clicker and learn to clicker train - you will be amazed at all the behaviors your dog can learn, and it's ALL mental exercise.  Your dog is telling you something - learn to listen and help him out.  Great way to do that is to get a copy of Turid Ruugas' book on Calming Signals, or if you want to be a real dog nerd, Barbara Handelman's book on body language. 

spiritdogs

I'm going to address your post one item at a time. 

My dog has recently started acting aggressive and dominant towards other dogs. Most aggressive behavior stems from fear, so please don't assume that dominance is the cause. 

He is just about 2 years old and has been going to the dog park several times a week since he was vaccinated and could do so.  Puppies that wait until after they have been completely vaccinated before they begin socializing often miss valuable social lessons from other puppies.  The American Veterinary Society has info on this here:  http://www.avsabonline.org/avsabonline/images/stories/Position_Statements/puppy%20socialization.pdf.  I encourage owners to get their puppies to class no later than 12 weeks of age.  When a dog hits age 2, earlier for some later for others, they are at the age of social maturity and may be ready to tell other dogs that they're tired of taking crap from them. 

He loves to be chased and has always acted very submissive when other dogs have caught him or surrounded him. It usually ends up with him on his back. It's possible that he *doesn't *enjoy this as much as you think, or that he is overwhelmed by it, or that he is now mature and doesn't want to do it any more - some dogs don't play for a lifetime.  Herding breeds often tire of dog play by the time they are mature.  Labs tend to play for a lifetime.  Even then, individual dogs of all breeds may surprise us one way or another.  Watch his body language - all of it, not just the rolling on his back.  Is he always looking back, or stopping and dodging and chasing other dogs back?  Is he always the one being chased?  One sided chasing may be a sign that he is a target for some of the other dogs' bullying.

The only time he has ever acted aggressively is when there is a frisbee involved and if a dog tries to take it from him he has snapped before.  Not unusual.  Dogs often protect items from other dogs.

He has never snapped at a dog prior to this past week for any other reason. These past couple of weeks leading up to the incident he would start to jump on dogs backs and hump them if they wouldn't chase them. I noticed this was mostly with dogs younger than him usually around 1 year old or so. This week he was getting chased by a couple of pit bulls which he usually loves and then they stop and one of them basically body checks him and then he just started snapping at it very aggressively (definitely wasn't playful).   What makes you think this is OK - if I were him, I would have been ticked off, too.  By the way, owners who let their Pits exhibit this body slamming behavior are asking for trouble.  It's bullying, and evidence that their dogs were never properly corrected for it by an adult dog when they were little.  That is not a park I would take my own dog to, and I have a hound who loves to play.   If you go to my web site on the Yappy Hour page, you'll see him playing normally with a Boston Terrier.  Normal play should never look like there's no give and take, or that one dog always takes the brunt of whatever game... http://www.pawsforpraise.com/Yappy_Hour.html  Scroll down to the link at the bottom.

A different pit bull but from the same owner was running along side the small dog park basically chasing the small ones up and down the fence and my dog started running along side him and then another small fight broke up. I couldn't tell if he started this one but the owner asked me "if my dog had a problem because her dogs never fight". Many Pit owners are quick to defend their dogs (can you blame them?) but fence running, and chasing smaller dogs can lead to "predatory drift" in any terrier, or indeed in any dog with those tendencies, and the first thing that a dog in that mode does when interrupted is often to redirect the frenzy on to the nearest dog (or person, which is why people often get bitten in similar circumstances - not because the dog is after them, but because they interrupted a predatory sequence of behavior).

He never shows any aggressive behavior on his leash or in the house towards other dogs and at the park it's difficult to reprimand him when he's off the leash.  If you are reprimanding him when he is just trying to survive, or defend himself or his possessions, you may be accidentally contributing to the problem.  There are many unintended consequences to punishment, and until people understand them all, it's best to stick to cues.  If your dog will not "come" immediately when you call, maybe he doesn't belong at the dog park until that skill is cemented.

Does anyone have any recommendations? He has so much energy that I couldn't imagine not taking him there.  Not all dogs are dog park dogs, and yet they still need exercise.  I think your dog is in a situation that will only lead to further aggression.  If you have noticed him really liking a few of the dogs, maybe you could ask those dogs' owners to form a smaller play group in someone's fenced yard or early am in a local tennis court.  Or, you can do what I do with my one dog who isn't that fond of large groups.  I throw the frisbee till my arm falls off lol.  There's a machine called Go Dog Go that you can teach your dog to use - it's a tennis ball launcher.  You can also go to classes and learn to do a dog sport, such as agility, tricks, rally, etc.  Mental exercise is much more tiring than all that running around, believe it or not.  Get a clicker and learn to clicker train - you will be amazed at all the behaviors your dog can learn, and it's ALL mental exercise.  Your dog is telling you something - learn to listen and help him out.  Great way to do that is to get a copy of Turid Ruugas' book on Calming Signals, or if you want to be a real dog nerd, Barbara Handelman's book on body language. 

I think the key with a dog like Wesker is to back WAY up and not even worry about other dogs right now.  I have two dogs that don't care to play with other dogs and are not appropriate for dog parks, but otherwise they are well behaved, well adjusted dogs (they don't react towards other dogs or care of other dogs are playing, they just don't want to).  Wesker sounds different, though, it sounds like he needs a major confidence boost.  He needs to be more confident with himself and with you *before* testing the waters around other dogs and working on that issue.  I had the same problem when Nikon was young.  He not only didn't want to have anything to do with other dogs but he would get worked into a frenzy if another dog was within 30 feet or so.  This is kind of a common phase for dogs of his breed and his lines, so I didn't really worry about it or push it, I just kept him away from other dogs and worked on regular obedience, playing, and bonding with him.  After a few more months, he was fine with other dogs.  I've had dogs come into our house temporarily or permanently and even take him to the park to meet my friend's dogs.  Over the summer we had three intact German Shepherds, none of which particularly care for each other up close, all playing with their toys and jumping into my friends pool, no one on a leash or paying any mind to what the other dogs were doing.  Wesker may be more sensitive than just going through a phase but I would have the same approach.  For now, manage him so that he never gets the change to react toward another dog.  Keep the focus on having fun with you and not having to worry about what other dogs are doing.  I really think for most dogs this is a confidence issue, like "I'ma get you before you get me" sort of reaction.