EDIT: I get it, the consensus is that this is incorrect. I retract my statement.
Surprised no one has said this yet:
This happens when you put zip-up clothing in the dryer. The fabric shrinks and shifts but the metal/plastic zipper doesn’t, creating a size mismatch that results in a bulge. Hang-dry your zip-up hoodies instead. I have hoodies that are close to a decade old and still haven’t begun to bunch up because they’ve never seen the inside of a dryer.
If the ship of Theseus were a meme