Well that freeze frame shows how my lower back has rounded out when I go quite low, but this only happens unloaded. Put a load on my back and I can stay more upright and don't round out. I appear to start to tilt forwards around a pivot point just above the start of the back rounding. Also this back rounding looks more apparent then it is due to over developed spinal electors.
What I really need to do is see a physio/coach that knows there **** and can point out where my actual issues are. The run of the mill physio I saw advised that apart from the over developed muscles in my lower back everything else was quite balanced, and my spinal positioning was good. It seems certain movements highlight my areas for development quite well