Echoing topendtorque's sentiments, any fatality is always sad especially when we cannot make sense of it.
Anyone who has flown a Bell 47 will know how low the main rotor blades descend when they are spinning after engine shutdown but before they are slow enough for the droop stops (aka Rabbit's Ears) to kick in. Sloping ground, or even a gust of wind, will bring those high intertia blades right down to head level.
It does not only happen in the field. It also happens at flight schools, as topendtorque presumably knows.