Self-inflicted damage from TNT (using flint and steel or flame bows) and splash potions cause friendly foxes to attack the player. Enderpearls don't cause this behavior, nor do snowballs; the latter don't do any damage or knockback at all.
Anything that deals damage and can be traced to the cause (i.e. a skeleton with a flame bow igniting TNT that hurts the player) will be targeted by friendly foxes. This includes other foxes, which is why they will try to kill each other.
Self-inflicted damage from TNT (using flint and steel or flame bows) and splash potions cause friendly foxes to attack the player. Enderpearls don't cause this behavior, nor do snowballs; the latter don't do any damage or knockback at all.
Anything that deals damage and can be traced to the cause (i.e. a skeleton with a flame bow igniting TNT that hurts the player) will be targeted by friendly foxes. This includes other foxes, which is why they will try to kill each other.