Maybe it was a Overspeed condition. If they had a governor failure or such during high power demand and it got away from them and resulted in a overspeed. Or undetected damage from previous unreported overspeeds.
It's in Robinson Safety Notice SN-36:
"Helicopters have been severely damaged by RPM overspeeds during liftoff. The overspeeds caused a tail rotor drive shaft vibration which led to immediate failure of shaft and tailcone. Throughout the normal RPM range,
tail rotor shaft vibration is controlled by damper bearing. However, damper is not effective above 120% RPM"
I'm doubting the main rotor blades went through the tailboom cutting off the tail assembly and continued as it did in the video without near-instant failure, as it did when the observed tailboom strike occurred.