Basically, you hit one zombie pigman, the entire nether is no longer any kind of safe, within minutes you will have hordes of zombie pigmen trying to kill you, I tested going 1,000 blocks away from the first zombie pigmen I ever hit and the zombie pigmen in the next area were still agressive. Basically instead of just the ghasts trying to kill me I have every zombie pigman in existence trying to kill me. I even wiped out all entities with /kill @e and the zombie pigmen that spawned in their place are still agressive.