Endermen are able to teleport themselves between dimensions, allowing them to safely travel from the Overworld to their home, The End. They are also very curious creatures, picking up and moving blocks around entirely to see what'll happen. Some endermen, however, want to know more than just "What happens if I put this bit of dirt by this tree." They want to see if there's another dimension, apart from just the End and the Overworld. These unfortunate endermen teleport themselves to the Nether, where they find a much harsher environment than anything they've seen before. And worst of all, the Nether, for unknown reasons, blocks endermen from teleporting out. The only escape is a portal, which they don't know how to construct. These lost, trapped creatures must to find new ways to live and survive. Some get lucky and discover a player's Nether portal, some group together and figure out ways to survive. But most simply don't adapt fast enough, and perish to the unforgiving wastelands. One enderman stumbled upon a ruined Bastion Remnant, and curiosity got the better of them. They saw these new blocks, and decided to take one. The piglin guards did not take this very kindly, and tried to kill the enderman. It teleported away, detecting the dangers. They thought they were safe, but they had gotten themselves into a much worse situation. This unfortunate enderman had teleported directly into a tribe of zombified piglins. It was all over for the enderman. A couple of hits and one ram from a zoglin, and the enderman was dead. The zombified piglin tribe left the dead enderman behind, unknowing of what they had done. The infection that kept them alive after death had spread from the zoglin to the enderman. They opened their remaining working eye, realized they were alive again, and stood up. They had become a zombified enderman. With the curse of being alive again, the enderman went back to wandering the Nether. They are no longer the curious enderman that entered. They are now a horrifying creature, destined to roam the Nether for the rest of their cursed life.

TL;DR
Enderman gets stuck in the Nether, gets killed by zombie piglins, becomes zombie.