Here's my theory, probably wrong lol....
Aircraft cruising, pilots in a not so-aware state, eating meals or such not monitoring system displays.
Hydraulic leak and fire occurs possibly from overheated brakes or some other problems, (modern jet hydraulic fluid is modified to be extremely hard to catch fire at room temperature) causing the flight controls to fail steering the aircraft to the left (westbound) thus forcing the autopilot to disconnect, pilots are fatigued from fire fumes (remember flight was at night while passengers where probably sleeping), while trying to set the transponder to 7700 they accidentally turn it off (quite easy) after performing procedures of aileron failure (trimming rudder ect) for stable flight and setting course for diversion airport, they fall unconscious and the aircraft continues to fly south-west until fuel becomes exhausted and crashes in the Indian sea. Of coarse much pilot confusion would've happened if these events occurred.