Horse Behavior? Help if you can?

very simple answer... animals are just like humans.. that's it.. all your questions are answered. let me give you an example... lets say a high school girl is confronted by 3 girls, one who is her friend... and one girl starts hitting her.. shes confused thinking that they are all after her trying to jump her so she hits all 3 of the girls.. just slight panic and confusion. Just as people "hate" on each other so will animals... could be anything.. one of the horses could have done/said something the other didn't like.. but you would never know b/c you don't speak their language... lol .. good luck!