TC, makes sense, given the higher operating rpm. Thanks. (But if you unload the tail (by going flat pitch as can be) while the main is at higher pitch ... you still run in to
V^2 and its inverse, I suppose.
TC, the other time the head/rotor blades can stall is at high speed, retreating blade stall, but the fix for that is usually a reduction in pitch and speed if one remains in control ... and so really doesn't fit this event.
As to VRS, that is usualy a power on condition, which again seems not to fit this event.
Question we may never be able to answer: what was the helo and the crew doing in the sixty seconds before it all began to go wrong?