The most likely failure was a sudden and violent jam in the transmission that took it from normal rotation speed to zero in a very short amount of time. That would cause huge counter torque that likely stressed the tail rotor/boom assembly to the failure point. Also break away of the the transmission/rotor assembly from the fuselage indicates shearing from both mounting points on top of the cabin.
Blancolerio has a very detailed explanation of this scenario too.
https://www.youtube.com/watch?v=U8Q8XuqlyMQ&t=323s
Exactly....