Spawning Piglins

Ever since I placed a Nether portal in my base suddenly Zombie Piglins have been spawning inside of it. They are not coming from the portal as I have blocked it off. Once they spawn I have noticed they are tending to wander to the slime chunk I am trying to build a farm in. There is only ever 1-2 of them at a time. I should also add my base is a tree base mainly consisting of oak leaves with some carpet, decently lit up.