I’ve always noticed this phenomenon on flights where the moment the seatbelt sign turns off, there’s a sudden mass exodus to the lavatories. The same thing happens on descent—especially when you pass 10,000 feet and hear the double ding. It seems like part of it is the “permission effect”: people have probably needed to go for a while, but the sign going off is the psychological green light. On descent, there also seems to be an anxiety component—people realize landing is imminent and that they’ll soon be stuck in their seats again, so there’s a rush to go “while they still can.”