Well, don't know why, but the Zombie pigmen AI doesn't act like what the other mob AI should work like.
What I expected to happen was...:
Once Zombie pigmen receive damage from other hostile mobs (skeleton, Pillager, anything that can deal damage to other hostile), they attack the hostile mob that deal damage to them...
What actually happened was...:
After the hostile got killed, the Zombie pigmen attempting to kill the player even though the player didn't deal any damage to them.
I've tested it for other mobs like Pillager vs skeleton, Spider vs Blaze, all of them work exactly the same way as they should, except zombie pigmen, which I think, that MoJang may mix them together?