I had shorts with pockets that made it feel like the phone was put away, but when I stood up they would catch the phone and tumble out.

A shirt pocket. I've dropped a phone in the toilet twice and in a bathtub once this way - it slipped out of the pocket as I was bending to wash my hands or pick up the toilet cleaning brush.

(I've since then stopped carrying anything in open pockets above the waist.)

Sit on a toilet and hold a phone up near your face (not over the front of the toilet). It’s easy to see in this situation how a slippery phone or sweaty palms could cause the phone to fall down and between your thighs.

Solution: Think some "shower thoughts" during your bathroom break.