Just my tuppence worth. The latest reports seem to indicate a plane flying along airways and my first thought was - bearing in mind their transponder was off and they weren't talking to anyone - thank goodness they didn't collide with another plane.
Some have suggested that an explosive decompression or even a slow decompression for that matter could have been the cause, losing communications - but surely anything disrupting the comms / transponder would also impact their ability to navigate?
Others suggest that some event - explosive decompression with failed oxygen would lead to hypoxia and this could have impaired their ability but how could they be so hypoxic as not to be able to enter the correct altitude in the autopilot but able to enter the details into a flight management system for a totally different set of co-ordinates?
Some are stating the pilot turned the transponder off as a precursor in a bid to commit suicide but then I recall hearing about the altitude change - if I pilot wanted to commit suicide, why not "go out in a blaze of glory" and charge along an airway toward oncoming traffic with ATC unaware what's going on? Surely, anyone intent on committing suicide isn't "rational" enough to switch from FL350 down to a level which would avoid any other traffic flying according to the Hemispherical rule.
Then why FL295? Because they were possibly transiting an area where the Quadrangal rule is in effect and they wanted to be sure that they didn't hit anything coming the other way, because this is in effect up to FL290 (see the Malaysian AIP). So why not fly at FL355? Possible because other planes flying at FL340 - 360 could see them pass relatively close under them and advise ATC of a "near miss"?
So when you add this all up together, I'm tending to think that this was a deliberate change of course, maybe a member of the crew had ulterior motives and had intended to divert elsewhere, for reasons unknown - who knows, maybe coercion of some sort or other? However he could only do this once his fellow (non involved) flight crew had actually left the cabin, for example for a toilet break, but what happened if he had miscalculated the fuel required to reach this new destination due to high level winds, due to the fact that he couldn't start his new flight because his co-crew didn't go for their break early enough, that the plane then was forced to ditch elsewhere?
I don't subscribe to the theory of someone else hijacking the plane by overpowering the crew because by now the backgrounds of all those on board would have been examined and anyone without a pilot's licence who would have been learning to operate a 737's navigational and comms system would be suspicious and I don't believe that an amateur, reliant on flight sims alone, would be able to adequately program a 737's FMS...