It was very unpleasant to watch that.
From the video, (although frame rate interference flicker often masks such things) it might have been a tail rotor drive failure. The reason I say that is that the tail rotor appears to briefly slow down and stop then restart. Due to the high yaw rate, it may then have actually begun to rotate opposite to its normal direction.
It does appear that the crew didn't react to loss of tail rotor authority (for whatever reason that occurred). First thing I'd have done was jettison the bucket then dump the collective lever. Most experienced, adequately trained pilots would have done those things.
What a shame.
