Heavily favored Big Brown was only successful at becoming the 12th straight contender to fall short of winning the Triple Crown at the final race since the last time it was accomplished in 1978. That's probably why the Triple Crown is so esteemed, because it's extremely hard to win. The winner of the Belmont Stakes was Da'Tara--the 38-1 longshot who nonetheless led the race from the gate to the wire.
Big Brown, whose success was practically assured by his trainer and people who know anything about horses, finished dead last. His jockey Kent Desormeaux said that when he was in third place and nudged Big Brown to one of his epic home stretch drives, the three-year-old just sputtered. "I had no horse."
Desormeaux pulled up when he realized that the win was not possible, knowing that getting the almost-champion back to the stables safe and uninjured was more important than whatever place Big Brown could achieve. The horse had a hiccup in his training in the days before the race when a crack was discovered in one of his hooves, but it was treated and a veterinarian said there was no reason why it would have contributed to the horse's poor showing.
When asked what could have possibly contributed to Big Brown's poor showing, if not physical injury, Desormeaux could only reply to a reporter, "I have no idea."