I'm with everybody else. Your form looks good. The only thing I see that might be causing the back issue is the exaggerated hyperextention of the lower back at the top. As a powerlifter you wouldn't want to take it that far anyway because when you do it makes you break at the knee and you lose the lift with the judges if they look close. For your purposes it may be that you're doing it intentionally for the max contraction of the lower back, but it could be what is causing the back pain. If it doesn't interfere with your goal for the lift, try stopping it at your upright standing position. Just a shot in the dark at stopping the back issue. Good luck with your goal!