Tail rotor failure. Watch again.
He performs a turn in one direction and it is immediately cancelled out by a reverse turn at the same time the TR slows down.
That 'X' section TR most definitely has stopped functioning correctly - it is simply free-wheeling.
Tail rotor shaft / bearing failure.
The flicker effect with the helo in the foreground is constant, the failed TR in the doomed chopper is all over the place.