On a server with just my friends, as soon as we enter the nether the pigmen get angry for literally nothing. None of us and no mobs are hurting them, we don't need to break blocks for it to happen either. Just being in their presence aggravates them
Once zombie pigmen get angry they stay angry until you can stay at least 40 blocks away from them for at least 40 seconds.
As long as you are within 40 blocks they stay angry and make other ZP within about 60 blocks of themselves angry.
Any angry ZP in unloaded chunks stay angry until the chunk is loaded again, so leaving the Neather doesn't help, they'll still be there when you come back.
That's the way it's supposed to work, there are reports that things aren't working completely normally, somebody claim that they teleported way across the Nether to somewhere they hadn't been before and the ZP there were angry, I don't know whether that is the case or not.
I'd suggest either tunneling under a lava sea to move away from them or, if you can pull it off, keep killing them until there aren't any angry ZP left and hope that that works.
If you take the tunneling route you'd have to know that there weren't any players staying behind near the angry ZP.