You can watch it here for free. http://espn.go.com/horse-racing/twinspires?wer
(Click "Race Replays" > Select "Hoosier Park" and the date "9/27" on the calendar > Select the 8th race) The thing happens at around 1:31 of the video.
Cliffs: As the horses turned for the stretch, the starting gate was still lined up across the track (they started very close to the far turn) and the horses had to be pulled up immediately. I guess there is no siren or any type of system that alerted the jockeys to the situation, so they were driving their horses up until the last few seconds.