I have a server with my girlfriend, and every now and then we have a mass pigmen spawn in the nether. Since they can go into portals they're in our portal room in the overworld and theres a load of baby pigmen on chickens as well and they're supposed to be rare since you don't really see them, I would love to tell you what causes this to happen but it just seems to be happening randomly and always crashing the server. Not seen it reported so im unsure on if it's happened to anyone else but its weird how we're seeing it and not knowing what's causing it I only loaded up this morning to see it like that.