Why? Because my sims are obsessed with back floating in the pool to the point they'll do it to exhaustion and drown. Once the first one drowned they started dropping like flies. I was just letting the...
I had one of my Sims drown due to being exhausted and deciding to backfloat instead of going to bed. It was during my first try at the Free Will Experience challenge. It happened during a thunderstorm too! All I could do is watch since I couldn't control her. It was so heartbreaking... :'( Backfloat is fine if you're controlling your Sims and they're not tired. But if they have full autonomy, some Sims just can't resist the incredibly strong lure of backfloating. They'll do it despite the weather and to the detriment of all their other needs until it kills them.
If you want to read the details of how it happened to my Sim, Whitney, here's a link in the spoiler.
For my second try with a group of eight instead of four, I didn't give them a pool just for this reason. BTW, I couldn't bring myself to save for this day either. Whitney is still alive and well in my custom template save along with her co-contestants and Ziggy.
Also, if you want to check out the challenge and try it yourself, here's a link to the thread on here: