High Altitude stalling is always a consequence of poor energy management, that is to say flying in the "T" thrust limited zone or "B" Buffet limited zone with a lack of situational awareness for the conditions, perhaps without the benefit of real time bank angle protection( No LNAV) or flying to close to "maximum" and then hitting turbulence or increasing warmer air and basically not descending, or limiting the bank. Mr Boeing,bless him says:
STALL RECOVERY.
In all upset situations, it is necessary to recover from a stall before applying any other recovery actions. To recover from the stall, angle of attack must be reduced below the stalling angle. Nose-down pitch control must be applied and maintained until the wings are unstalled. Under certain conditions, on airplanes with underwing-mounted engines, it may be necessary to reduce some thrust in order to prevent the angle of attack from continuing to increase. Once unstalled, upset recovery actions may be taken and thrust reapplied as needed.
Ref:
Aerodynamic Principles of Large-Airplane Upsets