Very sad to read that...could have been any number of causes for the accident, maybe he had a heart attack, maybe zigged when he should have zagged, maybe an equipment malfunction, maybe debris blew in his eyes and blinded him...
Sad indeed, but, and my first thought after reading this was zero-turns would be more apt to jerk out of control near a pond edge or cliff or other spots where the slightest move would make you zig instead of zag, unless one is totally focused. A steering wheel would be more forgiving.