The theory now is that her bones were not developed enough in her front legs. They race horses WAY too young. General consensus among responsible horse owners is not to ride a horse until it matures (around 4-5 years old). They run the races at 2-3 years old.
They also breed primarily for speed, and not necessarily for soundness, in many cases.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry