Regardless of how the engine stopped, the way the emergency was handled seems to be important.  It seems that flight in a pressurized aircraft can lead to some complacency born of comfort - Emergency O2 must be instantly available and utilized immediately.  At 21000 feet even a descent at 1000 fpm gives 20 minutes of flying time while the initial descent of 3000 fpm seems like an attempt to get to breathable air thus giving up valuable flight time.  If any amount of hypoxia and hyperventilation set in, any bad result is possible - picture the mind's logic operating like Swiss cheese - full of random holes unpredictable in affect.

 

I await the NTSB report.