An ex-pilot explains all of the available evidence with the simple theory that there was an electrical fire and the pilot was diverting to the nearest large airport. The media has made it out like the transponder was turned off by pressing a red button on the front of it. I was shocked that it was even possible to turn off the transponder. Apparently in an electrical fire, the standard procedure is to kill the circuit breakers for everything- which makes sense. The transponder is part of that because you have to be able to deal with an electrical fire in the transponder.
http://www.wired.com/autopia/2014/03...ectrical-fire/