My nether portal in the overworld is light up and gated to prevent mobs entering/leaving, but I recently was attacked by 10 or more zombies when I entered the nether. No skeletons, creepers or spiders.
Could it be a glitch where the pigmen are spawning as zombies? I killed 4 pigmen before the zombies appeared, and 2 were wondering the area after I slaughtered the hoard.
A big wave of pigmen with a single zombie attacked. Then a zombie villager appeared behind me in my small pigman hunting lodge which contains the portal. A couple more zombies appeared outside. This happened twice but the second time there were more zombies and less pigmen in the initial wave. I also had another wave without a zombie in my bunker.
I think it might be an event like the zombie hoards that attack villages.
Zombies have been dropping from higher platforms that can't be accessed from the portal platform and I have stayed close to the bunker so I doubt they have been coming through it. Especially because there has not been a single skeleton, creeper or spider.
Update.
I just saw a zombie spawn beside a pigman in a lit area. I had not hit any pigmen yet.
Snocrach also said:
Zombie pigmen can summon their zombie friends from the overworld to help
them fight the forces of evil.this is a normal minecraft feature – nothing to do with the world generation.
I am playing the Snocrash's Hermit Chunk Challenge in 1.9
Best Answer
I am not sure if this issue is fixed, but Zombie Pigmen can or could spawn Zombie reinforcements when attacked. It was intended that they would spawn Zombie Pigmen as reinforcements, but since the code is kind of related, I believe that could happen. It's possible that's still a thing in 1.9.