Might just be what I see about the race track they run on. But after the first pass thru by the entire group of horses it probably has divots and potholes in it, not to mention it's not the smoothest surface to begin with. To run a horse like that and put even 100 lb jockey on it who is whipping it to go faster. I have to believe the theory on weak bones is a myth. There's simply pushing an animal too hard and putting it on a surface that is hardly ideal.
Not too long ago, the NFL's Philadelphia Eagles closed Veteran's Stadium. The field was a liability for player's health. Case in point:
"http://en.wikipedia.org/wiki/Wendell_Davis"
"He is infamous for injuring both of his
patella tendons simultaneously while simply running a pass route on the notoriously bad
Astroturf at
Veterans Stadium in
Philadelphia. The injury was effectively career-ending."
I'd have to think any horse breaking it's ankle/leg is similar to any human athlete hitting a bad spot on the field just the same while running at 100% speed. The best you could hope for in that case is a twisted ankle, but expectations are realistically to break an ankle.