Klingon,
Sorry, I mean no disrespect to you and your perfectly valid input. I do, however disagree.
I really cannot think of any sensible cause. Whatever happened led to a high speed drop on to the roof with rotors stopped. It did not land, shut down and then fall through the roof.
I do so hope that there is enough evidence recoverable to enable the AAIB, for whom I have the greatest respect and admiration, to discover the cause.