Having assembled IKEA furniture, I'm pretty sure he didn't trap his sack during assembly lol.

Sitting on a stool with holes in it combined with a nice warm shower, scrotum relaxes, balls slip through a hole. The problem is that standing will try to yank both through at the same time, which just isn't going to work. I don't have balls of my own (someone who does please correct me if I'm wrong) but I'd imagine that after that startling moment, the scrotum would probably try to retract is contents for protection, which would make matters worse.