I don't want to open another can of worms but, if, for any reason, the mast and head moves with respect to the rest of the airframe the control systems WILL cause the blades to fly in un-commanded ways. Either the swashplate follows the mast and the rods that position it will cause unwanted control inputs or the swashplate stays aligned with the airframe and the links to the blades will move them in unwanted ways. I suspect that a small amount of mast relative movement could put enough control input to the blades/head to cause it to be ripped out of the airframe. This might explain the rocking motion seen by the observer before the head departed.
The same might happen if the control system gave a sudden un-commanded input if, say, something else let go that impacted or disrupted it.