is there a main cause for vets scratching horses before races?

Actually there could be a number of reasons vets may scratch a horse, minor or major. A horse who flips or strikes its head in the paddock or en route to the gate will unquestionably be scratched, that's a main cause without a doubt. The veterinarian will assess the horse’s overall demeanor and look for any indication that it might be favoring a leg or experiencing discomfort, as well as confirm it has vision in at least one eye, all possible reasons to scratch. The veterinarian palpates both front legs for swelling or heat and performs a flex test on each foreleg to uncover any area of concern that may be present. The groom then is asked to remove the horse from the stall and jog it in hand so the veterinarian can observe it in motion. With today’s emphasis on improving safety in racing, more stringent protocols now are in place and all for good reason. :)

(Your very observant, the role of regulatory veterinarians, although rarely heralded, is one of the most crucial in racing. It is their duty to increase the safety of horses and riders by ensuring that every horse that enters the starting gate has passed a prerace examination to verify it is healthy and sound enough to compete. But surveillance does not end at the prerace examination. Regulatory veterinarians observe the horses in the paddock and en route to the gate, studying them as they warm up and overseeing them during any mayhem that might occur while they are in the gate).

actually, it is the trainers who scratch the horses. A few reasons could be an unexpected lameness or illness. Another could be that the horse has no chance of winning and they don't want to put them through the extra stress of a race. Sometimes the track conditions can play a role (some horses don't run well in mud, etc.)

if the vet actually scratches the horse off the track it is because the horse appears unfit. other times a trainer does when a horse is lame or sick. If a trainer gets the 9 hole and in a tough class where the horse is burried, he can scratch the horse but he would have to pay a fine. but if he is scratched sick or lame they do not have to pay the fine. so often they are scratched lame when they arent lame at all. it is also a tactic when trying not to loose a horse who is racing in lower class claiming races.

edit: falsely scratching lame when racing in claiming races is to scare off potential claimers

Lameness

Unsoundness (lameness or disease of some sort)

Unfavourable or dangerous track conditions