In the case of the Airbus (and many other airplanes), wing/body design is a compromise of many factors.
The air at FL50 is very thin. At higher speeds, drag is increased. Drag that brings the nose up.
Gadalftb is correct, in this situation,with this airframe, more power/speed without proper retrimming, could cause a stall.
You are forgetting the active flight control system, which is controlling the flight surfaces and more-important than secondary factors from drag. (See Post #59).
You seem hung up on a secondary effect, when there was a serious primary problem with the all-important FBW flight controller. If the input to the FCS is garbage due to A/S sensor failure, then the output moving the flight surfaces will be garbage. The FBW alt. law message indicates that either most of the sensors were screwed up or the computers were. The later Flight envelope message strongly suggests that the output was indeed garbage.