For reasons yet unknown to us, the autopilot could not cope with the situation, and left the pilots in an unrecoverable position.
Be it autopillot or sensor failure feeding wrong information to the autopilot is another discussion.
This is rank speculation. What caused the AP to disengage and how Normal Law was left has not yet been revealed at all. Gysbreght's idea of Alpha-prot inspired panic followed by wrong decisions sounds a good possibility to me but I only fly couches nowadays.