Truly a sad story indeed. And not to make light of the situation, but guessing some operator error was in play? Mowing when too wet? or mowing on too steep of a grade right next to a pond?
Of course some kind of operator error played a part. But probably not an error that most people on here hasn't made at least once or twice (driving their tractor on a bit of a slope, driving on wet and lumpy terrain, etc.).
It's human nature to assign negligence to the operator in a case like this because we want to feel like we're smarter, more skilled, better prepared, etc. and that this can't happen to us. I don't think that's wise thinking.