This occurred with me the first time that I got around that weight. I looked at your video and (while I am not a coach) it doesn't really look bad at all. Watch where your eyes are, look more down. Focus just a touch more on your lower back tightness and push your hips back further. Get your hands more on top of the bar (your wrists are extended, not straight), and lower your elbows a bit. Back soreness can feel a bit strange but if it is a dull ache it is likely not an issue.