I've just read this input on an angling site that I visit.
Is this statement kosher?
All pilots are taught how to autorotate a helicopter in the event of an engine failure, this still enables the pilot to control the descent and pitch/yaw of the aircraft. Most helicopters have a centrifugal clutch fitted which will only engage the rotate if the clutch is spinning fast enough, this will also disengage in the event of an engine failure which allows the rotors to spin freely. I imagine that in this unfortunate case the clutch either failed to disengage or the gearbox was so badly damaged that debris stopped the rotatation.
Another possibility is that the rotor brake was on, or the pilot was pulling back on the stick which will slow the rotors.