I think given the lack of radio response, and the erratic flight patterns, the three best theories are:
1) A bomb went off in the rear of the plane, which damaged either the elevator and/or the horizontal stabilizer(s), as well as communications. This would mean the pilots had little control of the pitch of the plane. The ailerons were potentially unaffected, so the banking, descending turns were them trying something, anything, to get control of the plane. Without horizontal control, the plane either pitched up, which resulted in a stall and a rapid descent, or nosed down and just sped up until impact.
2) A pilot forced the crash, in which case the erratic flight patterns could have been either just that pilot's choice, or the result of some kind of struggle.
3) Explosive decompression due to something(a bomb, structural failure, etc. This was also a MH370 theory) which rapidly depressurized the plane. You've got only seconds at that altitude before you black out. Emergency descent procedures for most planes are steep, circular dives, similar to the plane's rapid descending 360. The pilots might have been trying to get down to a lower altitude ASAP and just blacked out.
I lean towards option 1 personally.