Icing most likely cause
A post above showed the track of the plane diverting left to Butte to skirt radar images of weather cells.
Also, Accuweather.com said that the plane passed through a layer of air at about 1500 feet that was conducive to icing because the temperatures were just below freezing and the air "Had 100 percent relative humidity or was saturated".
Similar icing conditions occurred with the recent Continental Airlines Twin turboprop that crashed at Buffalo New York last month. Also, the March 26, 2005 crash of another Pilatus PC 12/45 fatal to pilot and five passengers. Witnesses in each case say the planes seemed unstable and nosedived.
Former NTSB chariman JIm Hall said that one of NTSBs "Most Wanted" is FAA testing of the ability of turboprop planes to withstand a particular type of icing condition called "Super cooled Liquid Drops" before certification.
The buffalo flight pilots said they could not see forward because of ice buildup on the windscreen. This, plus ice-induced instability could explain the account of one witness for this Montana accident. That witness is a 14-year old CAP, who said the plane jerked to the left before nosediving. He said he thought he was watching a stunt plane because the pilot made so many turns. He may have simply been trying to see out the side windows. The plane was too high and to the right of centerline when it dived. The witness speculated that "He (Pilot) jerked the plane to the left too quickly and lost control of it, but thats just my guess". The pilot tried to pull up, but was too low. Airframe icing could contribute instability.
Ice buildup that is fast and thick from supercooled humidity could also affect the engine thrust as the propellor blades build up a coating and perhaps the engine ingests 100 percent humid air. Icing equipment, especially mechanical ones could not keep up.
All of this seems to indicate there may have been a severe and very fast icing condition that the very experienced pilot could not handle. He was 65 years old, a former air force pilot with thousands of hours, and flew this plane for the last four or five years for a fractional ownership company. The parents of some passengers were fractional owners, and the pilot was also a family friend.
I hope to never experience icing conditions that severe. I have had only one icing experience, and that was enough.