From the low RPM at the very end of the video I'd guess failure of the input from cbox to main transmission. With the engines suddenly being released of load they would tend to overspeed before systems caught up to control.
From the manual.
MAIN DRIVESHAFT FAILURE
INDICATIONS:
Left yaw.
Rapid decrease in ROTOR RPM.
Rapid increase In ENG RPM (N2).
Illumination of rotor RPM caution light with audio.
Possible increase in noise due to:
Overspeeding engine turbines.
Overspeeding combining gearbox.
Driveshaft breakage.