I would suggest sitting back just a little bit more. You want your shoulders to be directly over the bar at the start and you want your weight to stay on your heels. I'm guessing you might have done some Olympic lifting, which has different mechanics for the pull. One principle that applies is not rushing the first part of the pull. Pull the slack out of the bar to help emphasize tightness in your body. Also post a video with heavier weight because a heavier load will clarify deficiencies in technique.