Air Asia confirmed that the pilots requested deviation due to weather.
Suggests the airplane downing was weather-related.
It could be the aircraft was torn apart in flight from severe turbulence, or the engines flamed out and couldn't be restarted in time (icing, extreme water ingestion or other type of compressor stall), or there was a loss of instrumentation that the pilots could not overcome.
All three have happened before.
I looked at the times -- sunrise was at 5:49, and the last known contact was at 6:17 local. That indicates there was a visible horizon at the time of the crash, which helps rule out a JFK-Jr.-style disorientation into the ocean (though one pilot's disorientation and confusion doomed another Airbus, Air France 447).