Swede, they can die of accidents that you don't make or control. I have had some die of drowning.
Just out of curiosity, was this with story progression off or on?
If it really was off, then I gladly retract that statement, but I've always thought it limited the risks of uncontrolled sims dying on their own. All experiments I've done has pointed me to this conclusion, but since all probability reasearch could be faulty due to a statistical error, then I can never be sure!