Today the American Humane Society issued a statement about the probable causes of this accident that happened here in Santa Fe, New Mexico.

Investigations Indicate Movie Animal Accident Likely Result of Horse Instinct and Rider Unfamiliarity With Mount. During filming on Oct. 23, 2006, a quarter horse ran into a camera-carrying vehicle, severely injuring itself and its rider, Deryle Lujan, a professional rider/stuntman. Lujan was taken to the hospital in critical condition, and the horse's injuries were severe enough to warrant euthanasia on the set. The accident occurred in Diablo Canyon, west of Santa Fe, N.M.

Here's one possible cause—

The investigative reports noted that the horse had been trained both as a part of a four-horse wagon/stagecoach team and as a solo riding horse. In the scene in which the accident happened, the horse was being used for solo riding. It is believed, however, that during filming, the horse's wagon-team instincts may have caused it to cross in front of the camera vehicle in order to stay with or follow its "wagon mate," another horse (also being ridden solo) that was veering to the other side of the camera vehicle. When not being ridden, that horse was the "lead" animal in the same wagon team in which the ill-fated horse was the "wheel" animal.