I think you're staring with your hips a tiny bit too low, and its causing a very slight balance shift in your first/second pull. Your butt shoots up a small but noticeable amount faster than your shoulders do. So your shift from the hang to power position occurs a little late in the second pull and your hips have no choice but to swing at the bar a bit.
I think having your hips 1-2 inches higher in your start will help you get better tension in your back and butt in the beginning. As well, give some snatches with a pause @ the knee a try for warmups some time -- focusing on keeping your chest up, butt down at that knee position, and then feeling the transition to a more vertical torso power position before you extend