That's a really strange situation, I've never heard or seen horses doing that, although I suppose that it could happen. I'd think it's much more likely for the group to just "banish" the horse so to speak, and chase it away from them if it tried to follow.
I think what confuses me about your post is the gang mentality you say these horses had. I could see one, or maybe two horses picking on a horse, but not 6 of them at once. This sounds more like cainine behavior...
I don't know what to say about this. It just doesn't sound like horse behavior. It might be a stretch, but maybe the owner did something (drugs/scent sprays) that confused the horses and made them act like this. There are some really crazy people in the world.
Ditto on what Alwaysbehind said too. Never go into a field with horses without the owner's permission. I did it once, and although nothing happened to me or my stupid dog that I was chasing, the horses were sprinting around (thanks to stupid dog) and I could have been seriously hurt. For that matter, the horses could have been hurt too... I didn't know anything about them, they could have been recovering from an injury and not supposed to be galloping around madly. The lesson learned from that was keep a leash on the stupid dog. I assume everything was okay; It was my parent's neighbors and we are still friendly with them and they never said anything. I wouldn't blame them though if they were ******.