It's fairly simple. I foolishly hit a zombie pigman in the nether and all nearby zombie pigman turned aggressive towards me. Nothing weird there. However, new pigmen would keep showing up aggressive towards me no matter how many of them I dispatched, so I began to wonder if perhaps newly spawned pigmen were turning aggressive towards? I conducted a test where I turned the gamemode so peaceful, letting the mobs disappear and then back to normal. Just as I had thought newly spawned pigmen were still aggresive towards me whenever I came into range and would start attacking me. It's been a huge frustration as I now cannot enter the nether without constantly fighting.  I hope you will be able to find the root of the problem.