[shorod]

By your comment that it idled better than usual it sounds like it started back up? My initial thought was inertia fuel pump cutoff, but then it should not have restarted. A momentary open circuit in the switch should not have caused the engine to stall since there would still be some fuel in the fuel system, unless you had the engine wide open at the time.

I suppose my first advice would be to not take the bumps/tracks so fast, it can't be good on your suspension/wheels/back, or any trim pieces that are prone to rattle over time. If you weren't traveling at an unreasonable speed, then I'd be curious to know how many keys you have hanging from your keyring in the key cylinder switch. Maybe the switch or the ignition switch is getting worn and the jarring from the bump caused the ignition switch or key cylinder switch to momentarily open to the "Off" setting and kill the engine.

-Rod