There is a comment on another forum that uncommanded rudder is suspected. But even if the rudder was at full deflection could this not be counteracted and control maintained through engine power and aileron input?
Full rudder deflection at cruise speed can cause vertical fin separation. The rudder is sized to provide sufficient rudder power to overcome a worst case yaw deflection at or near stall speed. At cruise speed the rudder has enough power to fail the vertical fin. The rudder damper system is (usually) designed to prevent excessive rudder deflection at high speeds in a mechanical flight control system. In a fly by wire system that is taken care of in software. If something went wrong to cause a full rudder deflection at cruise, or worse yet rudder couplets (swinging the rudder at high deflection from one direction to the other) at cruise, the vertical fin
will fail. That was the cause of the loss of AA flight 587.