Can you be more specific with "back pain"? Where exactly is it? Sharp pain that comes and goes, or dull pain that lingers? Both sides?
Is your stance deliberately staggered? Your right foot is offset back about an inch from the left. You don't seem to be trying to get your knees out at the start of 275, but 185 you're actively shoving them out. Your stance looks a bit wide too. Bring it in about a half inch on either side so your heels are under your shoulders.