Fly by Wire problem.
Auto-pilot goes haywire from erroneous input data, pilot takes over but computer sets safety limits the pilot can’t overcome.
Airspeed read as too fast, auto-pilot cuts power, pilot takes over, tries to increase speed but safety limits built into the computer refuses to allow it.
Stalls due to attitude, falls.
Just a guess.
Unless Boeing’s design philosophy has changed, the pilot is always able to override the computer inputs by simply “hand flying” the airplane using conventional flight controls The auto throttles are not the autopilot. The auto throttles, if engaged, would sense an overspeed condition and reduce thrust, not “cut power.”
All of your power speculation has nothing to do with “fly by Wire.” Whatever happened will be determined after recovery of the Black, (actually orange) box. (actually a cylinder)
The Airbus someone mentioned will not allow the pilot to stall the airplane. Cheers.