If zombie pigmen aggro on a non-player mob, and successfully kill it, they automatically  become hostile to the player.

This can happen with skeletons and ghasts. If a skele or ghast hits a Pigman with a projectile, they will all aggro. Once they have killed the offending mob they will turn on the player, even if the player was not involved. This was not such a concern before but will become a bigger pain with the coming nether update, since players will be spending more time there.

Similarly, pigmen can walk through nether portals into villages. Iron golems attack pigmen on sight. If the pigmen kill the golem, they will then begin to attack the player as well.