Simplest theory I've heard yet was that the plane experience a fire with smoke in the cockpit. Possibly from the front landing gear. The pilots turned to one of the closest airports, with the easiest approach, Palau Langkawi. They set the automatic pilot and then started shutting off nonessential systems in the hope that the fire was electrical and they would extinguish whatever was burning by shutting off systems. (That's why the transponder was shut off.) Trying to extinguish the fire was the top priority before radioing in with air traffic control. But the smoke knocked them out before they could make any radio transmission. The plane flew to Palau Langkawi and flew over it and out to sea where the plane ran out of gas and hit the water.
Via Business Insider.