I've seen a few things cause this:
1. If you were on an incline, you could have come off the seat just enough for the safety system to cut off the deck, but not long enough for it to shut down the engine.
2. Your seat switch or PTO switch could have a 'bad' spot in it, from age and use, that could be creating a random problem.
3. Due to age, the RIO switch on the reverse pedal linkage could be getting slightly engaged, when the pedals return to neutral, just enough to make contact to shut the deck off randomly. Adjusting the switch usually fixes that.
4. I can pretty much rule out the coil going bad on the PTO clutch since it stayed on for an hour after that. When the coil starts going bad, they usually work for an incrementally less period of time every time they cut off.