RVDT, I have heard it said many times about the RW, that transition instruction is more important in this type than just about any other, particularly with regard to DRO.
The pilot in the video attempted to correct out the roll-over, instead of dumping the collective which probably would have saved the ship.
Either pure lack of type training or poor skill.
[Edit] - I just watched it again with the sound on - sounded like the rpm started audibly dropping well before he got into trouble.