What happened can be split into two answers.
First, the second portal was created
I have the same problem in my world. There is a portal in my overworld that keeps "re-appearing" no matter how many times I take it down (note, it always appears at the same location, which I believe is significant). I'm assuming it is due to one of these two "quirks":
Portal generation can be a bit funny at times. Let's say you have an overworld portal and travel to the nether. If a portal cannot be found within a certain distance from where the game wants to spawn it, the game will keep trying, moving the portal further and further away until it finds a place where it will fit. However, if you travel back through the portal, the "original" overworld portal may be too far away. This causes the game to re-create a portal somewhere else in the overworld (I constantly hear reports of people returning to the overworld in a dark, scary cave, rather than in the portal they made at a "safe" location).
Another possible cause, due to the way "portal travel" is calculated and the fact that portals are two blocks wide, it is possible to align a portal so that going through the left side will bring you to one exit portal, and going through the right side will bring you to a second location. I have not observed this one in my own world, but it is possible this quirk may also lead to a second "random" portal appearing.
In both cases, even if you didn't walk through to the second location causing the second portal to generate, perhaps an item or another mob did, creating the second portal.
Second, Zombie Pigmen will sometimes randomly spawn from portals
This is likely what you observed. It is possible that a Pigman walked through the portal (yes, mobs and items are able to travel through portals) but they usually don't like to walk through portals unless pushed, and moreover, they won't walk around (and therefore won't walk through the portal) in the first place if you aren't in the Nether with them.