The restroom stall was already occupied when they went in
Going to a public restroom can be an unpleasant experience in itself. Nevertheless, if you have to go, you have to go, and there is nothing anyone can do about it. Just make sure to check the stall first.
That way, you can avoid situations like these. We wonder whether the person could hold it in long enough to change toilets. They might have been afraid to disturb the eight-legged creature that was walking on the door.