Watch how you bring your shins to the bar. You rock forward on to your toes. This puts you in a very bad position to pull and almost guarantees rounding of the back. Stay in the middle of your foot. Bring your shins to the bar by bending your knees and dropping your butt. Once your shins touch, don't drop your butt any more. Right now your hips are far too high. See if that helps. Also, lifting shoes are your friends. Wear em.