When hitting a Zombie Pigman he becomes agressive but does not make surrounding Pigmen (all within less than 10 blocks of the Zombie Pigman the player hit) agressive most of the time, usually it will take several hits of the original Pigman to make others agressive. Also newly spawning Pigmen are rarely made agressive by other agressive Pigmen within range.

Additionally when Pigmen are spawned using /summon PigZombie ~ ~ ~

from a command block they are only actually agressive towards the player around 50% of the time.