They don't know yet. What's so weird is, it took 10 minutes to descend. he could have just jammed the plane into a nosedive, but he did a pretty gradual descent. Also, the recording box recorded the co pilot breathing normally the whole time--no speaking, no indication of, well, anything.
it sounds like he had a farily long absence from flying at one point, and investigators are now focusing on that piece of his past.