Having viewed all the videos, photos and links on this thread, I am unable to see any obvious external faults with the aircraft or its handling until the turn is initiated while in an effective hover at the top of the ascent.
Given that the engines would have been operating at or close to maximum power in sustaining the hover, is it possible that the effectiveness of the tail rotor was critically reduced just as something distracted the pilot just for a second while the rotation rate increased?
Is there a preferred turn direction? Would the aircraft have survived if the turn had been made in the opposite direction? Is the tail rotor authority sufficient to arrest a spin in either direction while in a high hover?
It seems to me that with no forward velocity, once the aircraft started spinning, there was simply no way to go other than straight down.