Actually Bob the accident report did address that question. They believed the previous pilot flew the aircraft in the exact same condition, and that he exited the cockpit with the now undone crotch strap lying loose but still routed through the seat handle, Sean didn't pass the strap through the handle, it was already there. On recreating the scenario the examiners noted how easy it was to reach down and grab the strap without being able to see the mis-routing.
No need for ill-feeling over this, it was a tragedy and lessons will be learned. RIP Sean.