It could be a tail rotor failure, a tail rotor drive failure or loss of tail rotor effectiveness (LTE). LTE occurs when the rotor is subject to winds that prevent the rotor from carrying-out it’s function; think of something akin to a stall. My money is on a drive failure.
This is as expected from the AD:
Investigators found the pilot’s pedals became disconnected from the tail rotor and caused the aircraft to make an uncontrollable right turn before it spun and fell… The pedals and tail rotor are linked by a mechanism which failed after parts of it had become disconnected and there was a “build-up of black grease” on one component. The failure led to the pitch of the tail rotor blades being changed “until they reached the physical limit of their travel”, investigators said.
