The skeleton over-spawn issue in 16w41a brought to light... Zombie pigmen who are hit by a skeleton or stray's arror fired at a player, aggro to player(s) after killing the skeleton...

this is inconsistent with other mob damage directed at the player, pigmen do not aggro on (non-reflected) ghast fireball damage, creeper splash damage, or blaze shots that hit them on their way to the player

note: I've heard maybe this was a bug before that was squashed and returned?