Gina,

If you watch the first rep slowly as you break the floor you start with your hips too low and chest too upright. Your hips quickly shift back and up and the bar flies off the floor. The second and third reps you fix this a little as you start in a better position. Unfortunately in powerlifting only the first rep matters. You look at those start positions on 2 and 3 and think about what they feel like. Try to get there without the bar guiding you into that position on a descent.