A stall in an airplane is related to the aerodynamics of the wing (angle of attack too high or speed insufficient to produce adequate lift) and has nothing to do with the engines.
Almost correct. A stall is solely caused by the angle of attack between the wing and the relative wind exceeding the critical angle of attack. When this happens the airflow separates from the top of the wing, causing the aircraft to lose most of its lift. The aircraft is now falling instead of flying, and is in an aerodynamic stall.
While low airspeed can contribute to a stall, it isn't the primary cause of one, as an aircraft can stall from any combination of airspeed and attitude if the critical angle of attack is exceeded. An example would be severe turbulence where an aircraft is forcefully pushed up; the plane may be momentarily stalled due to the rapid change in the angle of attack even though it is at a normal cruise attitude and airspeed.
A stall is a safe maneuver for a trained pilot in a properly loaded plane. It is usually first taught within the first ten hours of a pilot's training. But if the load shifted in as extreme a way as it possibly did here, the aircraft's center of gravity would have shifted so far aft of the flight envelope that it induced a stall that was impossible to recover from.
Palarran's explanation of the stall is more detailed and accurate than mine was as I was just trying to keep it simple but believe me that an aerodynamic stall is a function of the airfoil not the engine.
