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.
He was sitting on the stool, and with hot water all around things got loose and slippery enough to pop through one of the holes you can see in the picture.
I work in hospice, so I'm in the homes of many, many elderly people. While it's not just the elderly, some people have balance, strength, endurance issues, or injuries that make standing and bathing risky. Falls are bad, falls when naked, wet, and onto hard surfaces are worse. You can purchase specialized medical equipment meant for shower use, but I can see someone using an inexpensive plastic stool as an alternative.