Prior to the spin you were bearing away in a gust. Maybe a runner caught or your blades were a bit dull and the back end lost traction?
The spinning seems to have come on very suddenly as you seem to slide out of place in the cockpit...
After looking at the slo-mo version a few times I don't see anything that looks like your rear runner caught anything. Maybe the front runner.
I think Don has it pretty well nailed. You start hiking, ease the sheet, and turn down. It looks your body started to slide to the outside of the cockpit just
before the runner broke free. If you hit the cockpit side that could have helped initiate the the spin to the outside. All that would have been helped if your runner wasn't really sharp.
How windy was it when you spun?
Glad to hear you're OK.
Cheers,
Geoff S.