The story that accompanies the photos of the bleeding states “Stewards rushed to inspect the horse and found that he had bitten himself on the underside of the tip of his tongue.”.
So the cut is on the underside of his tongue apparently. Perhaps from having his tongue pressed against his teeth? If she warmed him up in a similar fashion as the photos from yesterday, I could see that happening. Every pic showed his lips and mouth gaping open with tremendous rein pressure.