The foster I have that is a young male Rottweiler is now showing no incidents of aggression towards people, but now aggression towards certain dogs.

There have about 4 incidents now and if he meets a dog (always onleash) he is great with 99% of them...even if large male dogs growl, bark and posture at him he is Mr. Calm. He doesn't react and just stands firm, ears up, nubby tail up and chest out. He has never reacted negatively to countless large dogs barking,growling up a storm to him.

The weird thing is, when we meet a dog that cowers, tries to hide behind its owner, darts around all scared like it can't wait to get away from him it ignites something in him and he focuses/fixates with this stare...he growls, lunges and wants to go after the scared dog. I was talking to a trainer about this and he said (without seeing it happen) that my foster dog is a low in self confidence and his way of feeling powerful is reacting strongly to dogs that he knows are "weak" that he could bully. He said he doesn't react to the large dogs that posture towards him b/c he knows he cannot "win".

Does this make sense???

i don't know about low self esteem-

imo-some dogs try to "take out" weaker dogs .
This isn't good for a few reasons.

Bart started out like that. He was only aggressive to very skittish nervous dogs. Then he started being aggressive to other dogs too.

Plus-a dog needs to realize that it is not his place to decide what dogs need to be taken out. It is yours.

No mater how unbalanced another dog may be-only the pack leader gets to give the go ahead to deal with that.
And because you don't want that to happen-the dog needs to just do nothing.