While I'm not familiar with the horse, I could tell immediately why the horse refused to compete and tried to toss his jockey. And no, he was not hurt.
The track was basically a swimming pool - it was deep mud - and some horses just won't run on mud. He slipped a bit coming out of the stall and apparently decided that no, he was not risking it. His jockey made the sensible decision not to push the matter.
The ultimate reason: All of the horse's previous dirt track experience was in the UAE. Where, ya know, it doesn't rain very often.
Here's the official video so you can see a horse saying NO with all its might for yourself.