One possiblility is the deck may have been pretty slippery. A little bit of oscillation could have been enough to "unstick" the skids, and with Nr not yet up to full speed, he wouldn't have enough tail rotor authority to control the spin.
70% N1 is the normal point at which to switch on the generator, and the pilot states that he subsequently rolled on full throttle as they fell, so I don't buy the "starting on full throttle" scenario - which sounds more like a mistake that could happen on a piston helicopter.