An idle thought, prompted by some of the ideas submitted on this (and other) threads:
In gliding we are taught the concept of the 'Yates effect' (named for aerodynamicist AH Yates), which goes something like this:
Why gliders speed up when contacting lift
You may have noticed that the ASI shows a speed increase when you fly into the core of a thermal. The reason was first described by Dr AH Yates. An up-draught will increase the angle of attack because the airflow comes more from below. Since the lift from the wings always acts at right angles to the relative airflow, the lift acts further forward and accelerates the glider.
One wonders if what goes for gliders is also applicable in an airliner flying at FL350 / M 0.83. I know from personal experience (stubble fire plume entry, for example) that the Yates effect can be huge: a glider may require a rapid change of pitch of >+60 degrees (!!) to avoid exceeding max manoeuvre speed, shortly followed by an equally aggressive (zero 'gee') recovery to a 'normal' pitch attitude to avoid the stall.
An unusually severe 'seat-of-the-pants' lift force experienced at the same time as an (assumed) unusual attitude and loss of airspeed references - followed by (speculation) an incorrect recovery to normal pitch attitude - *might* have initiated this upset.