Well I don't mean to turn it into a front hack squat. But your upper body should not be paralell to the floor. This places huge stress on the back and pretty much takes the legs out of the equation. By being somewhere in between vertical and paralell to the floor you should find your strongest position where your legs are lifting in conjunction with your back.

I looked at it again and it seems as though you are pulling with youlr shoulders quite in front of the bar. I think this is part of what is pulling you forward and forcing you to extend your legs and bend down more. I believe your shoulders should be in line vertically with the bar.

IW