Tumbling?
If the rotors were stopped early on in the sequence of events, then there's nothing much to stop the helicopter tumbling in response to the last manoeuvre and aerodynamic forces, as the constraints on its motion from the reservoir of rotor angular momentum has been removed. Helicopters tend to have lower moments of inertia than fixed wing aircraft, so could more easily tumble. If it was in fact tumbling, rather than just being seen spiraling in projection, then it argues for a rapid, high loss of rotor speed.