I am going to guess a flight control linkage failure.
There is nothing in the rotor drive system that would input a yaw rotation as in a single rotor helicopter with a Tail Rotor drive failure.
I will watch it a couple of more times and see what stands out.
If it was a swashplate failure I would think the pitch attitude would have not remained as flat as it did.
As best as I could tell both rotors were being driven....as if the Synchronization Shaft or one of the gearboxes had failed the blades would have done themselves in and that would be very visible along with the bits and pieces of blades and aircraft being scattered all about.
As best as can see it....it appears the blades stay in phase during several screen shots which would suggest it was not a rotor rotation problem.
They were able to keep the aircraft reasonably level until right at the very end when it looks as thought the nose is beginning to pitch up while the vertical rate of descent is increasing which could also mean the aft head was not responding to a increase in power or up Thrust Lever (Collective) and the forward head was.
That should make me think the problem was in the aft head....either in the flight control linkages or a mechanical failure of an actuator...or a swashplate failure of some kind.
AFCS/SAS does not have the authority to input a yaw input that cannot be over ridden by the Crew.....in normal situations.
There have been a couple of total upsets in flight that caused the Crews to lose control of the aircraft and in one case it was thought the aircraft had done a complete roll.
This is one account.....not that it has anything to do with crash in Idaho.
Also....my comments are just guesses.....and are not anything but that.
http://chinook-helicopter.com/Flight...x_May_1998.pdf
My heart goes out to the Crew.....they were flying that thing to the very end.