Sounds like this dog may have had problems recognizing people. Like a dog that sees a truck moving in the distance and lunges after it behaving like it was a prey object moving right close to it. It almost seems like an eye sight issue but really it's the way the brain processes the information. Did you get him from a breeder? If so did you ask whether other puppies from this breeder had similar problems? Was this breeder known for producing AB with aggression? The AB I've seen didn't have people aggression.