During the supposed fall that Billy McGinn took. You can see the rider is pulling extremely hard on the right rein. This is done to literally "bull-dog" the horse. It obvious that the ride has the horses head pulled hard to the right. That will always cause a horse to fall.
While generally speaking it would be an error for Quincy to use the word "autopsy" instead of "necropsy" to describe the examination of the horse, this was probably done deliberately so as to not confuse the audience. However, it would be pointless for Quincy to do it, since as he is not a veterinarian, any findings he might have found would not be admissible as any kind of evidence.