Moving Armor Stands

I'm not aware of the official name of the mechanic, but when you hit a mob, it can possibly hit another mob if they are close enough. A Zombie Pigman got into my house and I attacked it, but it also attacked the armor stand that was near if and moved it a few blocks away from it's original position. I think this should be a bug because armor stands shouldn't move in Minecraft Single Player Survival from this effect.