Sorry, the question that I meant to get out was, "What did the trainer say about it?" :lol:
The scenario that you described above is very typical of dog parks, and, in my experience, where 99.9% of the fights that happen at dog parks begin. I have to admit that I don't do dog parks. I just worry about fights, disease, weird people who can intentionally hurt your dog, etc. Instead, I arrange play groups with friends whom I know and trust. That way, I know that they're taking good care of their dogs and it is understood by us all that If we see one of our dogs getting out of control that it's okay for another to step in and reprimand. The other benefit is that our dogs see the other dogs as an extension of their packs and the other owners as authority figures, rather than total strangers. Having said this, even in our own back yards, scuffles tend to happen at the gate as well from time to time (when a new dog enters). Because of this, we all hold our dogs back away from the gate, allow the new owner and dog to enter, take the leash off the new dog and then allow the new dog to enter the main yard where the situation is more likely to be controlled if a fight does occur. Then, each of us releases our dogs one at a time to go greet the "new guy" so that he/she doesn't feel overwhelmed.
The situation at the gate is one of dominance. Your guy is telling the new guy, "I'm the boss here, get it??" Which generally isn't harmful. . . As long as the other dog doesn't decide to contradict your dog's view of his position of authority. Then a fight can ensue, which can be dangerous to your dog, the new dog and any other dog and human that decides to get into the middle of it. It can also cause what is called "the pack mentality", which could result in all of the dogs ganging up on the new dog, which has the potential to have awful consequenses.
On your walks you will want to teach him to focus on you, rather than the other approaching dogs. This will require much training and I know that you've taken quite a few training classes with your guy, but training is a life long lesson that never stops. Especially for dominant dogs. Ask your trainers about teaching you how to train something called "Attention" or the command, "Watch me". Refine it behind closed doors, work up to the actual situation, and then, use it in every day life. If you see another dog approaching, walk off of the sidewalk and into the tree lawn or someone's front lawn, make him sit by your side and tell him, "Watch me." By this point, you should have refined this command to the point that he knows that he will get a reward for ignoring the other dog. Eventually, you will want to work this command up and into your heeling exercises, so that your dog is walking by your side and giving you full attention at the same time, even as you walk past another dog.
As for the reasons, your wife's pregnancy is definitely a possibility. Our dogs sense these things and he may be guarding her. It could also be that he's guarding your little girl. (My 4 y/o got pretty protective over our 1 y/o when we first brought her home.) My guess, however, is that he's just at that age. He's a teenager becoming an adult, so he's starting to become more dominant. I think that we tend to forget that dog aggression is a completely natural thing in wild dogs, some moreso than others and we need to train and socialize the domesticated dog away from their natural instinct. Sounds like you've done a great job with his puppy socialization, I'd just keep up the training.
Hope this helps!