Very tough to take.
I lost my old Labrador to his back going out suddenly. It seems he had an undetected weakness in his back that had been there some time, so long that a 'bone bridge' had formed to try to compensate. It worked, until his foot had an issue and was wrapped, and he was having to hop in the back. It was a surprise when he just couldn't get up off the floor the next day, not in any pain, but completely paralyzed. Xrays showed his back was broken and he'd not walk again. He had a ticking time bomb there.. a little weakness we'd never have known about before it went off. Perhaps this incident with Barbaro, and your horse were similar. A leg is only as strong as it's weakest link. :~\
It's true you just never know what could be behind something like that. I'm sorry to hear about your dog, some things are just freaky.
Our horse was only 4 years old, the vet said that type of injury is usually seen in race horses, but he had seen quite a few like ours that were just the result of a bad step, possibly on a small rock that rolled as he stepped on it or something like that.
I just can't help but think it's possible there was a tiny hairline micro-fracture of some kind before the "wrong step" Barbaro took.
One story mentioned in passing that he had gone to his knees at some point pre-gate - but I'm also thinking that was pre-Derby maybe. I didn't see it - but I also was asleep, not on purpose - grrr - during the Preakness run-up and awoke with a start just as the leg must have snapped.
On TV, Barbaro was limping sideways towards the rail and the other horses were running on - I thought I was seeing a replay and had slept through the whole race - but it *was* the race. Dazed and confused, I couldn't figure out what had happened for a few seconds.
Sorry you lost your furbaby - I know how bad it feels.