I can see why you have lower back pain. You are much too loose going into the bottom. You're very uncomfortable with the proper back angle and you want to bail out with your chest up as soon as possible.
Really focus on staying tighter than tight and do not relax going into the bottom. Reach back with your hips while staying tight and balanced mid-foot on the way down. Do NOT lift your chin on the way up. Drive hips straight up.