Educated guess - the speculation of a thrust lever clutch failure will be correct. One engine throttle failed to move in response to the autothrottle command at level off (bad clutch, or high downstream forces due to a seized pulley or kinked push-pull cable). Crew was rusty, possible distracted by something else, and failed to notice the throttle split. Aircraft rolled due to the thrust asymmetry, crew caught off-guard by sudden AP disconnect, lost SA, and did all the wrong things. Specifics of why the crew failed to react appropriately to a relatively benign failure will remain a mystery unless the CVR memory module is located.
Critics and lawyers will try to blame Boeing (deep pockets - unlike the operator), ignoring that the autothrottle defect had existed and gone uncorrected for some time, and SOP for the pilots is to monitor T/L position and engine parameters.