there's an interesting article in Wired magazine that there might have been a fire. Which would explain the change of course, as the plane seems to have been headed to the closest airport. The article is very comprehensive, but then again, there are other things that are not explained, like with the cellphones that kept ringing.
The whole thing is very mysterious and leaves a lot of room for speculation, obviously. You try not to be influenced by all the media hype around it, but they're not making this very easy...