The aircraft was on a training exercise hovering or manoeuvring over rocky terrain (it was in the Pyrenees) when the tail rotor clipped an outcrop of rock. The loss of control resulted in the aircraft falling near-vertically for a considerable distance down a mountain gulley/re-entrant before being destroyed in a post-crash fire. The crash was non-survivable for all 3 on board.
Surprisingly the accident was witnessed by a Britsh Forces mountain climber (RAF engineer?) who provided eye-witness accounts which allowed the BOI team to locate the blade strike marks on the rock caused by the tail strike and reconstruct the accident events.
To answer your questions, they were conducting Mountain Flying training and the direct cause of the accident was the tail rotor strike. If you want to know any more, then contact the Aviation Standards team at Middle Wallop and if you have the appropriate credentials, they may answer any other questions you have.