Thinking on what Slap has said, maybe there was a hijacking unrelated to Islamist terrorism or politics of any sort? Say, one or more of the crew members were part of a conspiracy to steal something on board and/or to kidnap (a) particular passenger(s)?

Part of the heist crew is supposed to be waiting at Penang to offload the goods and the pilot(s), but for whatever reason, the plane doesn’t end up landing there. If the crew had put together a SHTF plan, they change course, dial in the autopilot, and bail out over land, leaving the passengers aloft.* If not, they begin looking for somewhere else to set down.

*Though one would assume that passengers would have made phone calls after the hijackers exited, assuming they were not all suffering form hypoxia related to depressurization related to the hijacker(’)s(’) jump. But that invites the question of why no cell phone calls were made in any case. Maybe the hijacker(’)s(’) bagged them all up first thing? If they had thought to do that beforehand, I expect they had a Plan B in place.