If you pay attention, there may be some subtleties that might give you a clue. Dogs pay close attention to body language. He may see the Collies as a threat because of something not easily noticeable, and perhaps he is generalizing if one of them has given him bad vibes and the others look similar enough for him to consider them a threat. My boy really didn't like small white fluffy dogs, I have no idea why....but perhaps because many were noisy and wanted to get in his face.