johnnydoom Usually an injury, whether sudden onset, or previous. I never raced a "lame" horse, but others would.
That is another thing the judges are required to do, watch for obvious lameness prior to the start of every race. You must 'warm up' your horse 3 races prior to the one you are in, and make at least 1 lap on the track. At this time, the judges are all looking at every horse on the track for signs of being 'unfit', and also checking for unreported equipment changes. They do have sharp eyes, as they are supposed to. Still, shit happens.
Thoroughbred horses do not 'warm up', they go 'cold'. They only get loose during the post parade, and may be scratched by the judges at that point if they detect something wrong.