Pilot error does not appear to me to be the most likely cause. Airframe break-up due to severe turbulence/overspeed?
Overspeed reulting from A/H failure in cloud? CO affecting occupants?
I assume they would have been able to recover from a spin in the VMC conditions where the hillwalkers saw them.
I agree that a report of the aircraft descending into the ground, as opposed to flying into it more or less level, is interesting information as to the conduct of the flight. Scud running is less likely to result in steep descents - 'cause you're not high enough to do it
Any of the quoted circumstances (other than the CO) would happen only as a result of failure to maintain speed, and/or attitude. That would take us back to pilot actions. An airframe will not break up in turbulence, if the aircraft is flown at a margin slower than Va. A spin will not be entered if speed and attitude control is maintained. A/H failures happen, but can and should be managed.
It is possible for a pilot to loose control of at aircraft in IMC or no practical vis conditions, with JFK Jr. being one of the more memorable examples.