*Background*
We've had our dog for about four years now and according to the shelter where we got him, he's now seven years old. He's always been a very sweet, gentle dog both towards us and family and our four guinea pigs which he licks and has open access to. He has had issues from day one as we believe he was beat and/or constrained. When we first got him just a raised voice would cause him to crouch in fear and pee himself. He's gotten over that but is still very, very sensitive to our emotions. He doesn't like to be on his leash or in a small area, he gets very nervous and defensive but to my knowledge has never bitten anyone/anything. He's very excitable/has a ton of energy. You'd never know he's seven and has two bad knees (one replaced). He is also incredibly intelligent. He knows the names of the guinea pigs, has learned our routine exactly, knows each toy by name, etc.
*The problem*
For about three months now we've been taking him to the dog park. For the first month he did really well. Got along with all the dogs, never showed any signs of aggression. Around the second month mark he began displaying dominance through mounting behavior. Most of the dogs (and owners) didn't mind and he quickly rose to "top dog" of the park. This didn't seem to be a problem until the last couple weeks he's began being aggressive. When other dogs basically bother him (ie chase him or try to play when he didn't want to) he would growl or snap, but never actually bite. As if he was saying "back off." Well tonight there was a small dog there which was barking and snapping at any bigger dog that came near it. A few of the other dogs were playing very rough and decided as a group to go after the small dog. When the barking and yipping started my dog got involved and as we were all running to grab our respective dogs it ended up that my dog had his mouth around the small dog. He didn't use any pressure, the dog never cried there were no marks or anything, but what would make him do this? And to change his behavior so fast? I obviously can't take him back to the park if I know this could happen again. It just seems so out of character for him. If he accidentally hurts my he "apologizes" by snuggling up and giving me kisses.
Any thoughts about this change in behavior?

