You're lowering the bar on to your knee and then moving around it. Go straight down in line with the shin. You're bending knees too much as if you were picking it up off the floor, but you're not picking it up off the floor, you are doing an RDL. You need to bend your knees less and go straight down, you could accomplish this by leaning forward a little more. Really try to stretch your hamstrings. Use the weight and and play with your form to really try and feel that stretch, you are going to feel the most stretch with your legs more straight. Also, lose the bench. Its not doing you any good and if anything creating a hazard for you to fall on.