Pushing a pig or other mob into a nether portal will get them stuck in an endless loop of going through the portal. They don't have time to move away from the portal before teleporting back. This happens both in and out of the nether. Baiting them with an item they're attracted to doesn't help. I suspect it could work in multiplayer if a player on the receiving end already has the bait out but in single player that isn't an option. The only fix I could think of was pushing a minecart containing the mob through.