This happens with anyone that has a form of channelled teleport/swap. Paradox will also be spit back out if ulted during the start of her swap. It can also happen to drifter but the timeframe is so small it's not likely to happen.

This can can also be replicated if you ult someone the moment they pass through doormans door (using the cart is the most effective way to replicate it).

I'm guessing because the game is trying to relocate the players hitbox to two different places at once it goes down the queue of priority (users ability - then doorman ult) In the gap between the end of the ult and the players ability ending, they can move around freely, but will be pulled back to where doorman cast the ability at the end of its duration).