Immediately thereafter the aural
warning for autopilot disconnect (named Cavalry Charge) was
activated. The disconnection was also confirmed by FDR data. The
warning sounded continuously during the following 18 seconds.
When either pilot disconnects the autopilot the calvary charge aural will only alert once for about 2-3 seconds.
When the autopilot disconnects automatically because of a failure it will continue to sound the calvary charge aural until a pilot manually selects the autopilot disconnect switch and that will silence the aural.
It sounds to me like the autopilot disconnected automatically.