The only things I see here is you're dropping your hips just a bit when you squeeze up, and your back is unlocking. Keep your hips up when you squeeze up - your hamstrings should be so tight they sing. THe only way to fix that is to lift at a weight it doesn't unlock, keep adding weight, and for you to MAKE IT STOP HAPPENING. Ironically, the first thing usually fixes the second, if you squeeze up so hard your hamstrings are tight and your arms are pulling agains the bar.