Most likely terrorists on board at this stage...
-They have found out two pax travelled with stolen passports.
-Some reports seem to indicate a distress call mentioning fuselage damage.
Now, that's just me, but it was a flight whose majority of pax were Chinese, the flight was code shared with China Southern I believe. Malaysia is a pretty good place for islamist terrorist cells to operate due to the local affinity for hard-lined Quran-bashing.
That's just a hunch, but that's where I'd put my money.
In the meantime, may they all rest in peace and my deepest feelings to the families and friends.