Just out of interest; does the auto pilot disconnect anyway 'automatically' at 200' or thereabouts if it senses idle power etc. or would it have been the crews lives flashing past and the F/O who manually disconnected it?
The autopilot does not automatically disconnect at any height and in fact after landing, when autolanding, will remain engaged until the aircraft stops on the runway unless manually disconnected by the pilot.
BA38's autopilot was engaged in APP(roach) mode and was flying the ILS. With insufficient power the speed was inevitably decaying.
Reasons for an autopilot disconnect can be but are not limited to an input made via the control column by either pilot OR a pilot pushing any disengage switch OR, as mentioned above, perhaps the autopilot system reaching its trim limits.
For the autopilot to disconnect itself after a cursory glance I have not readily been able to find any reference in my manuals regarding what parameters would have to be exceeded for that to happen but I presume the autopilot system would have such parameters.
Edited for technical accuracy following BOAC's comments.