[PyrNet-L] Aggression

Tonight was number 5 of 7 puppy classes. Sam did incredibly well tonight- he had one of the best nights in class since we started! He did as told, behaved, smiled, belly rubbed, down-stay (more or less... has a way to go there...) and after class I was talking with the instructor about the next phase, and she was enlightening me on the merits of the Gentle Leader. The next class was due to start in 15 minutes, and enter a Golden. Sam did his *big boy* bark and growled at him... the Golden and his handler went on into the room and started down toward the corner of the room where we had been working. The Golden was more or less ignoring Sam- I think he turned and looked once. Sam didn't like that at all, and charged this dog (stripped the leash out of my hand) and stood there cornering him and his handler barking and growling and in an aggressive posture, and I went after him- pulling and tugging him to break up the altercation. They did not get physical......(thank God) and I guess what puzzles me is that he has been in a class with other dogs and there has not been a problem. He is around other dogs- albeit not alot, and there hasn't been a problem there either. So why this dog? Was it a territorial thing- where *that is our corner- you can't have it* except that it started at the door when the Golden first walked into the room...... the whole thing was so unexpected that it was frightening for me as well as the other handler, who really fought to hold off her Golden...... and the Golden was not the only other dog in the room either. Any thoughts?