you can definately see the guy trying to undo something at the rear corner just as it lifts off though... the individual strap might have been long enough (especially if loosened by the ground crew chap) to allow the heli to move a small distance before becoming taught.
I imagine it snagged and then popped loose, hence why the heli seems free afterwards, but not until it had already halted it's ascent which then caused a tail strike (in slow mo you can actually see the blade stop on the second touch) resulting in the spinning movement.