If you look at the aerial shot from
Denver plane skids off runway - Yahoo! News Photos
See image 14 of 34
You will notice some quite distinct phases of the incident.
At the start there is a linear departure from the runway. That is whatever caused the aircraft to depart to the left had occurred a way back on the runway. Whatever it was did not cause further deviation. The aircraft kept going in a straight line. (This would tend to reject theories about differential braking/thrust)
Shortly after leaving the runway the aircraft starts into a radius turn to the left. Causes for this could be as simple as higher drag on the left hand side - perhaps deeper snow on the left?
A little while later the aircraft crosses taxiway WC and possibly straightens up. Definitely a short while later it loses its undercarriage (note the widening of the track) and commences a straight run to its stopping point.
My theory is that a single catastrophic incident occurred on the runway causing an immediate turn left. There was no further dynamic effects until the aircraft encountered snow/terrain. At this stage either the crew started braking with faulty brakes or the differential drag from the terrain caused the curve. Finally, the undercarriage was sheared off and the aircraft continued in a straight line till it stopped.