Need to work on holding your back in extension--esp. the upper back. You let your upper back relax right before the pull, and then let it relax more partway up (hence the weird little hitch that you have in most of the reps when the bar gets to around your lower thigh).
Really embrace step 4 (chest up) of the deadlift...don't pull until you've taken out all of the slack. And really focus on keeping your entire back extended and rigid the entire rep.