Looks like it was during shutdown as those blades have only struck once as it has rolled over, unless someone was very quick on the throttle.
The tail boom looks like it has been hit hard on the ground - enough to snap it - and then scraped the broken end on the ground as it yawed/rolled over.
Unexpected input to the control is a definite possibility given the ground is flat, level and looks firm.
Wondering if inadvertent aft cyclic just after selecting idle on the engine might have enough control power to tip the aircraft back quickly enough to smash the tail in.