Bryan and Justin really nailed it on the head!
-Starting with your butt too low and subsequently your shoulders directly over the bar instead of in front of it (with scapulas over the bar)
-You are Jerking it off the ground instead of squeezing it off the ground.
The jerking the bar off the ground is probably what is leading to the lower back rounding more than anything as the yanking of the bar jerks your body out of alignment. Fix these things and you will be golden