spiritdogs
It's also wise to consider that he did do more than bluster and actually connected, which is not necessarily as accidental as we might hope.

It is possible, which is why I've made a note that he may be becoming intolerant of other dogs at his age, but I have seen situations where puppies (just like Boxers...and one I know of personally WAS a Boxer...lol) that were punished by older dogs, squirmed and wiggled and pretty much caused self-injury when a gentle muzzle grab turned into a tug session by the dog underneath. A lot depends on simple physics of the direction of force and which animal was going what way at what speed. Kind of like when I was rammed in the face by Shimmer at agility not so long ago. My behaviour on its own was normal, and her behaviour on its own was normal, the two were performed all the time at different times, but the two put together at that moment added up to a facial injury.

Causing injuries to a Boxer's muzzle wouldn't be that hard - puppies have sensitive skin, boxers have extremely short hair, and there's a lot of "flap" to Boxer faces. Some Boxer folks I know have said their dogs get small injuries much easier than some of their other dogs. I'm not sure if they as a breed have more easily-injured skin, or it's just anecdotal. I just know there's been a bit of a trend, so I think it's interesting to note.