Some (not all, and as far as I know not the initial pigman) pigmen agro after one get's hit by a blaze and start attacking the player.
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I've found this as well, (random aggression) and I think it's because of social spawning of zombies in the nether. A pigman gets hit by a blaze, and it's dark enough that social spawning spawns a zombie. This zombie is aggressive towards the player, and I'm not sure of the specifics, but it seems that the zombie is somehow denoted as a pigman, so the game thinks that the player has aggro'd a pigman. It could be fixed by preventing zombies from spawning from pigmen, if this is indeed the problem. I hope this helped.
Here this video might help confirm it:
https://www.youtube.com/watch?v=vokGKpkYV_Q
Unable to get Pigmen aggro'd (two turned hostile to me for apparently no reason, the rest of the group staying neutral); however blaze fireballs are definitely hitting (and doing damage to) nether mobs, something that hasn't happened before.
If a bug, possibly related to fix for MC-68740.