hard to tell what your knees are doing in this video, but looking at your feet/hips/low back it seems like the knees must be sliding (third rep especially).
you're not setting your back before pulling, either. you get into a decent position but you don't lock it there. obv you'll get some amount of rounding on a 3rm, but typically the back will stay mostly flat until the bar comes up the shin a little and it rounds a bit. your back is at it's roundest point immediately as the bar breaks from the floor.