First of all, tell the person holding the camera to take a few steps back and stop moving around.
Your back was not neutral on any rep. You aren't getting tight enough as you setup either, it looks like you got your lats somewhat tight on the first rep but on the second one they gave out as soon as the bar left the floor. You are simultaneously trying to pull your shoulder blades down and pull the bar off the floor, don't rush yourself.
Do strict RDLs for sets of 8-12, stay a few reps away from failure and don't allow any back rounding. Also rows, I would recommend barbell row (not pendlay rows, don't touch the floor to maintain tension on your back). High reps again, maybe even 10-15, no cheating.
Don't listen to anyone who tells you to tuck your chin, this has been addressed by many coaches, most notably Boris Sheiko and Bryce Lewis.