What you have is improper breeding, horses with lighter bones in their legs run faster, and their bones break easier.
This horse was sired by a horse that had a short career, due to ankle problems.
As breeders become aware of these problems, they will favor bloodlines with stronger bones.
They can also do MRIs that will reveal microfractures, and I think you'll see more of this.
A couple of rules changes may be indicated, if a horse refuses to get into the gate, they get forced into the gate, but this seems to indicate that there may be something wrong and the horse doesn't want to run, maybe they should just scratch horses that don't want to run.

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