Basically when you kill a zombified piglin in 1 hit (i.e. with a crit and a smite 5 netherite sword or something) the others get angry.

How to reproduce:

1. Find a group of zombified piglins.
2. Get a powerful sword enough to kill it in 1 hit.
3. Kill it.                                                                                                                                       The other zombified piglins get angry. This may or may not apply to other neutral mobs.