While fighting a group of Pigmen, I was engaged in combat by a few Zombie friends who were traveling through the Nether (somehow). I don't know exactly when or how they spawned, but they started attacking me when I punched a pigmen. 

Possibly related to this reported bug from last year: https://bugs.mojang.com/browse/MC-14800?jql=project%20%3D%20MC%20AND%20resolution%20%3D%20Unresolved%20AND%20text%20~%20%22zombie%20nether%22%20ORDER%20BY%20updated%20DESC