Originally Posted by
six30nine
Assuming it's the same thing that's happened to me recently, I'm going to guess it's a combination of your chainring not being perfectly centered/round and/or your pedal stroke not being completely smooth at the high cadence you were at. I suspect this combo results in the chain tension "pulsing" at the right amount & frequency to cause the chain to slip or drop.
If you had had anywhere near the correct chain tension, neither of these would be a legitimate problem.