Every horse pins their ears. It's how they express their attitude when they're not happy with something. Our one gelding, Columbo pins his ears, but only at certain times. He does it when we have the theraputic program running and when there are volunteers on either side of him and leader leading him. I've come to the conclusion that he hates when there are people all around him, in his space. I would too honestly. We only put the very independent riders on him so we don't need someone on his right and left sides. He's perfectly fine with the leader though. He never gets aggressive!
If a horse starts to get aggressive, than that becomes the problem, not their ears. The aggressive behaviour needs to be corrected immediately, and constantly. Be very consistant, that is the key.