Zombie Pigman attack anything that attacks them, while sending a broadcast to others to attack the attacker as well. But, I recently found out that if you 1-shot the pigman, the other pigman will not go after you, even if they are literly 5 blocks away. Basically, the pigman are ignoring the damage you did to the dead pigman.

Simpler: Pigman ignores 1-shot dead pigman. Goes on with life as if nothing happened.