I don't think that it needs too much imagination to answer the 'how' part of your question. There have been dozens of discussions on this forum (I don't want to start another) arguing the pros and cons of the pilot leaving the cockpit with the blades at high rpm - either under power or immediately after shut down.
As to the 'why' part, I'm not going to break the first rule of Pprune by speculating. But again, it doesn't need too much imagination here either. This isn't the first time that someone has been hit by the m/r blades.
The official report will give its findings in due course.
TRC