Or was the aircraft excursion on Plan A, skirting Indonesia and Thai airspace and then something else happened that resulted in a turn on to south?
Postulating: the aircraft was under control for the first part of the excursion by, let us suppose, a hijacker. Then, while heading NW, something happened to disable the hijacker and this resulted in the turn south. At this point there was no competent pilot available to control the flight.