Why has the Moonraker Shuttle Carrier Aircraft crashed in the Yukon?

Moonraker came out two years before the first shuttle flight and only shortly after the first shuttle ferry, so some of the logistics of ferrying would have been surmised by the writers given information available at the time.

According to the Wikipedia article you helpfully cite, the SCA, when loaded with the Orbiter, can only fly about 1000 nautical miles before needing to refuel. Initial testing was done at 450 mph and at a maximum of 26,000 feet, and a trip from Edwards Air Force Base in California to Kennedy in Florida would have been done in two or three legs, depending on weather. Eventually NASA would test and reject aerial refueling as an unnecessary risk, but this was done post-Moonraker.

NASA analyzed 11 years worth of weather data from a multitude of locations, then ran probability analyses that took into account fronts, squall lines, thunderstorms, precipitation and the possibility of VFR flight - and remember this was in the days of punch cards and magnetic storage tapes - to plan the safest flight route with the least turbulence possible. This information and those first tests would have been about all the writers of Moonraker had to draw from, though I have not done an exhaustive search of literature at the time, and other scientific publications may have made applicable predictions.

The only way across the Atlantic in a short or medium-range aircraft is a North Atlantic route that refuels in Iceland (in fact, when the Endeavour did its trip to the Paris Airshow, it did just that, leaving from Goose Bay, Canada and stopping in Iceland and the UK on its way). So any eastward flight the writers might have concocted would include a Canadian stop unless they assumed aerial refueling was possible. Since the pilots were predicting an early arrival to England, it seems they were not expecting to make any stops. Perhaps they considered aerial refueling over Canada (less populous, less crowded air space) and a route that was over land as much as possible (the Canada - Iceland route).

This still doesn't fully explain the Yukon, though we quickly jump from the Orbiter exiting the SCA to Bond frolicking on another aircraft, so perhaps something happens to the trajectory of the SCA once the orbiter exits. The SCA cannot fly properly without the Orbiter unless it has 8000 pounds of ballast, so a wayward crash landing is perhaps conceivable. As much of this film is a stretch, is this one stretch too many?