The latest expansion of the search area vindicates my thrice deleted posts suggesting searching along the extrapolated IGATI - Pulau island track (basically extrapolate the last known track westwards until fuel exhaustion).
That, and post from @Tarzanboy most plausible to date....
What if MH370 wanted to perform an emergency landing for whatever reason. Close to IGARI point the crew entered a possible airport to land in their FMS which could be VVCT CAN THO with VOR "TRN", because this one has a 3000m runway which is close to the intended route ahead. But, there is another "TRN" VOR closer by, guess where: TRANG VOR close to the Andaman sea. The crew under severe stress executes the top TRN (closest by) in the FMS and the plane turns immediately to that point. Could this explain the hard left turn after IGARI point towards the Andaman sea?
I once watched a BA 747 turn 50° after an incorrect waypoint input. It occurred in darkness and the flightcrew were completely oblivious until asked to confirm their position (heading east to the South Pacific off the New South Wales coast when they should have been tracking to Sydney over land). I can imagine a crew under pressure easily making a similar mistake with no visual references to alert them to a navigation error.