My guess is that he is territorial (sometimes, you, or even the space around the dog is considered "territory") or simply "guardy" and reactive, as many Aussies are. Some of them simply are NOT everyone's friend, nor will they ever be. I believe you got bitten because you were frustrating his desire to "get" the guy or the other passersby. Interfering with a dog that is in a state of aggression can easily earn you a redirected bite, even from your own dog. I would not walk this dog in public without a muzzle, as he could do damage to someone and earn you a lawsuit - and I would complete the protocol in "Click to Calm" http://www.dogwise.com/ItemDetails.cfm?ID=DTB825
Another possibility is that the dog has a seizure disorder, or as someone else suggested, a thyroid problem. But, if the aggression is triggered by the appearance of strangers, I'd bet on "guardy", not ill. If it seems idiopathic, then maybe a vet visit to have the thyroid checked.
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