I heard two logical reasons that make sense to me. Too much inbreeding/linebreeding (so many horses are bred from the same winner horses) and they say this horse was raced too young - never was able to develope it’s bones/ligaments properly.
Both of those are pretty accurate. All Thoroughbreds’ birthdays are on January 1 of the year they were born. So hypothetically, if Eight Belles were born on December 31, 2005, she would have been registered as having the birthday of January 1, 2005, and would have been 2 1/2 years of age instead of 3 years old.
Fast legs are rarely sturdy legs, also. The sole quality looked for in race horses is the ability to race, so sturdiness is rarely a quality that is bred into these horses.
Having breed Chesapeake Bay Retrievers for hunting there's certain dogs that appearing in almost every pedigree, I'm sure it's the same for Thoroughbreds.