That was not a Tail Boom Strike by a Main Rotor Blade.....that was a Tail Boom SEVERED by the the Main Rotor.....a significant difference in my book.
As to the. cause....something major to cause that amount of downward flexing or tilting of the Main Rotor..
A small clue for you pprune accident investigators.....the close proximity of the severed bits would suggest it happened pretty close to where the rest of the aircraft fetched up.