Accidently trimming by hitting the wrong switch?
A. The auto pilot will disconnect if you use stabilizer trim.
B. If you fly manually, you will feel this after a few turns of the trim wheel. It would be very weird if you continue to trim and fight the out of trim with the yoke at the same time.
Unless the PM was trimming? That would be weird too.
The little CVR data(if true) we have implies one pilot did something the other didnt exactly like.The PM makes a call to ATC(about diversion so probably more than momentary call) but he mistakes the transmit switch for the trim switch.If this mistake was made,he would be trimming down when he thought he was transmitting to ATC.Theyre opposite each other on the stick.
If the PF is flying manually there is no AP disconnect wailer to let the PM know the error of his ways.Environmental noises(wshear rain) may camouflage the trim wheel noise.PM isnt holding the stick so is unaware of the forces on the stick that he is unintentionally creating.Only PF.If this situation is not corrected almost instantly by PF( trim cutout switches/remove PM's hand from stick) then at this altitude,it could be fatal.
Icing is not the cause.99% sure of this.
All conjecture but this had to be catastrophic flt control problem.Whether pilot induced or not.Vmca encounter also possible.