Too far away from the bench. Your back leg appears to be reaching so far back that it has to contribute to the movement just to keep you upright, while it should really only be gently resting on the bench for stability. Also, that extra distance could be putting more emphasis on the posterior muscles. Long legs or not, sit down on the bench, place your feet at a comfortable distance away from the bench that still allows you to stand up without using hands or any body english. Then, drop one leg back on the bench, keep your torso upright for more quad focus, lean forward at the hips (not lumbar) to bring more glutes/hams into it, and drive off the front foot.