Darko,
When she was left running 'at idle' was that ground or flight idle i.e. 25,500 or 43,700 or were you not able to tell with all the electrics gone?
If there are no electrics the engine is designed to run on and can only be stopped with the emergency fuel shut off. If this fuel shut off cable had been damaged in the crash then you might not be able to stop it as you describe.
When UK pilots hover the Gazelle they do so no higher than 2 meters (5 feet) because if you are higher than that it is difficult to effect an engine-off landing from the hover (perhaps what you describe). It will more often than not lead to excessive rate of descent as the rotor is low inertia and you will lose lift fast as the NR decays.
It could be anything from a partial FCU (fuel control unit)failure, clutch failure, or any number of other things that could have failed causing the incident, that is for the engineers to determine I guess.
The important thing for you is that both of you survived and walked away!
Good luck,
HEDP