Per the Telegraph (UK)(quote seems to be from my link above) - Malaysian authorities CAN NOT say why they are expanding search area.
It's probably related to intelligence data provided by a third nation ally. China, the U.S. and other nations have that region covered with electronic monitoring from resources on the ground, air, land, sea and from orbit. I'm guessing someone looked at super-top-secret pings of some kind and said "pssstt...look over here, but don't tell anyone we told you."