A frame by frame clip shows a sudden yaw and the tail folding to the right. The rotor system appears to be under power at this point. The severed tail is struck as it drifts above the falling aircraft. The gearbox is the last to depart about a third of the way down.
On the surface it could be a catastrophic transmission failure.
There seems to be enough of the wreck remaining to come to a conclusive finding as to the cause.
Until then, it is just speculation.