Your back does look like it is folding a bit at the bottom of the squat. Pay attention to the endcaps of the barbell. Notice how the bar moves forward at the bottom? This typically comes as a result of losing back extension or you "losing tightness". Really focus on getting tight at the top. Proud chest, lower back in extension (think butt out, or whatever gets you to do it), big air and clench your abs. I would also like to see you get your chest down earlier.