Looks like something really freaky occurred. According to radar records the plane essentially stopped and fell out of the sky.
https://au.news.yahoo.com/world/a/25875755/airasia-flight-qz8501-search-and-rescue-underway/
No expert here, but if there was engine failure and a massive headwind or some other reason for the plane to lose thrust, then there would be no lift and the plane would just fall.
I'm sure you know but to just spell it out, the turbines suck in air, compress it then ignite it to generate thrust. The thrust drives the plane forward and wing design makes the air flow under the wing faster than the air over, generating lift. Should engines conk out there should still be enough forward momentum to put the plane into a glide, generating enough lift to slow decent, but if there air thins out or some other reason to lose lift then it is just nothing more than a hunk of metal falling out of the sky.