It's very hard to do any analysis with the full accident report. The sentence "AIRFRAME FAILURE-IN FLIGHT" means that the airplane broke into pieces prior to impacting the ground.
The accidents that I've read about with that happening, the cause usually is flying into a thunderstorm, some sort of disorientation of the pilot so the pilot put the airplane into a high-G maneuver and the wing/tail broke off because it was overstressed. Also, it could be a mechanical failure where an autopilot failed, the pilot didn't recognize it in time. Or, it could be severe turbulence in the area and that overstressed the airframe.
These are just some of the possible reasons, an accident can have many causes. It is not necessarily one event or cause that would cause the accident.
ps. After adding the section in the next post, it looks like the pilot made a serious error: flying into instrument conditions that he was not rated for.