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.
This works as intended.
When zombie pigmen come to the aid of one of their kind, it's because the pigman being attacked broadcasts the call for assistance. If the pigman is killed with one hit, it dies before it can make that broadcast, so there's nothing to alert the other pigmen.
Simpler: Zombie pigmen don't keep an eye out for attacks on their brothers; they just listen out for calls for help.