Is it possible the filly incurred stress fractures during the race that snapped immediately afterwards?
We had a guy that performed a bad parachute landing who tried to gut out. A couple days later, he was doing a 10 mile run with his company when the fibula in his leg snapped. X-rays showed stress fractures on the bone that were probably due to the bad landing.
I remember watching the wrester Sid Vicious making a move and landing on an odd angle and it looked like his ankle turned into rubber, course its no secret those TV wrestlers are pumped up on steroids, makes a person wonder what these athlete horses diet is.