You were rolling off the throttle when it happened, so I am guessing rear slide due to engine braking. Anytime the rear loses traction it can step out, and any time it steps out you can highside. Highsides do happen under braking (although much less often than under throttle). If I recall, there was some pretty dirty asphalt in that corner, so maybe the engine braking in combination with the low traction was enough to do it? What kind of shape was your rear tire in in the middle section?
Still, no matter what, pretty bizarre crash and definitely some bad luck. Heal quickly!