Birdstrike into the engines, perhaps unlikely to be a cause - what about birdstrike into the fenestron?
If the cab had been in the hover at the time, I can only imagine that it would have a rapid and severe effect on controllability. Any violent yawing and pitching motion could easily give the impression of "tumbling" when seen from a distance at night. Any autorotation initiated from this position with no forward speed and a high yaw rate would have a limited chance of success.