Well, according to the initial FAA/NTSB report, the pilot had over 8500 hrs of civilian time (unknown amount of military time) and over 2000 hrs in the Pilatus PC-12. The pilot was not related to any of the passengers.
Two of the families on board were the daughters of the aircraft operator (not pilot), and it was unsure if the aircraft was being operated under Part 91 (General Aviation Rules) or under Part 135 (Charter on Demand). Technically, if an aircraft is listed on a charter operators DO-85 list of approved aircraft, than ALL of it's flights are supposed to be operated under the more strict Part 135 rules (with the exception of a company executive being on board and the flight being for business purposes).
The Pilatus PC-12 does not normally have any "black boxes" on board. So if there is a CVR (Cockpit Voice Recorder) or a FDR (Flight Data Recorder), I would be suprised. Very few of those aircraft are so equiped, as it is NOT required equipment.
I have flown the PC-12, and can attest its it's airworthiness. But like any aircraft, it has it's limitations. According to early reports, there were obviously more people on board than seats (14pax vs 11-12 seats, depending upon configuration). Weight may have been an issue upon departing it's last fuel stop, but by the time it had arrived in Montana, that should no longer have been an issue. Even the CG (Center of Gravity), as previously mentioned, is a wide envelope in this aircraft.
Mountain flying has it's own unique set of challenges, which I'd have to believe a pilot with 8500TT/2000 in type would be aware of. This doesn't make anyone infallible, but it certainly lends to the belief that something most unusual did occur in this case.
It is not uncommon when flying into smaller airports with limited ATC (Air Traffic Control) communications and radar, to cancel IFR while still in the air, prior to landing, provided VFR conditions existed. As they had diverted from BZN to BTM, and had cancelled their IFR flight plan while still airborne, the pilot must have felt that a safe landing in VFR conditions was assured at the time.
Given what has been issued in the initial report, my guess would have to be LLWS (low level wind shear). While the aircraft was low and slow on short final approach, a wind shear of greater than -15kts could have created a very serious problem, and could have possibly resulted in this unfortunate incident.
Of course none of this explanation can make up for the loss of these people. They were the friends and family to many. Though no explanation can make up for their loss, I only offer my thoughts to help provide a little understanding of what can happen in this type of situation. and most certainly my thoughts go out to those friends and families of the victims.