Two things led to the death (directly) of these people:
1. Lack of experience in landing on trolleys - hence his OVEr reaction every time he compensated for the dollies movement.
2. Not wearing a proper crash helmet.
As soon as he overcontrolled - driving the tail rotor into the tarmac, the chopper spun and the G forces smacked his head up against the cockpit rendering him unconscious.
The rest is contentious.
No-one fought any fires, no one rescued them. No fuel tanks errupted into a ball of fire, no FAA rules killed them.
Possibly one of the most tragic accidents I have ever seen, watching fellow professionals die very slowly in the most bizarre way??