Accident reports are full of "Controlled flight into terrain" causes. Many by experienced pilots.
Mountain flying is dangerous, weather can move in rapidly. Even the best pilots can get complacent and think they can make it.
Throw in a possible mechanical failure with the aircraft or it's avionics and it would not be a good position to be in. A mechanical failure could even be an exhaust leak into the cabin and Fossett being overcome by carbon monoxide.
Lots of possibilities and we will probably not know exactly what happened.