From the information given so far, it appears possible that a proper transfer of control between the two pilots may not have occurred as it should. How much if any this may have contributed to the outcome is yet to be established. Other factors not yet fully considered may have contributed as well.
For line pilots the following explanation is quite unnecessary. Others may find it worth consideration though.
When one pilot assumes control of an airplane from the other pilot there is a transfer of control protocol which should be adhered to. Most airlines and other operators of multi-crew aircraft have their own written SOP containing a specific series of words and actions to assure that a proper transfer of control takes place. (the following assumes that the autopilot is not engaged)
Representative examples of a "Positive transfer of control" SOP would contain language to the effect of: Pilot assuming control clearly announces "My airplane" or "I have control" while placing their hands on the flight controls. The pilot being relieved of control verifies that the other pilot is on the controls, then releases control and states "Your airplane" or "You have control".
Exact wording of the SOP will of course vary somewhat between operators but some method of assuring that someone has control is necessary!