That weight looks much too light to draw any inference from. You might not even be able to move the way you are right here at all on a 5rm.
Here's my $0.02:
It looks like your positioning is wrong because your hips are way too high. You can see that when you have a flat back, your shoulders are quite far out in front of the bar. When you hyperextend your back, which you appear to do before you start, at least on this very light set, then it appears as though your shoulders are close to the correct position. However, you're artificially shortening the back segment by hyperextending it, and your leverage would thank you if you just lowered your hips a bit more.
Have you watched the 5 step deadlift video? If you have, I think your problem is in step #3, bringing your shins to the bar. FWIW, I'm 6'3" with the torso of a 7'0" man (just kidding, but you get the point), and I had a similar problem to you. Not sure if it's at all related to anthropometry, but I would lean forward off balance to bring my shins to the bar. In order to start the pull, I'd have to bring my shoulders back and get into balance before the bar would leave the ground. In doing this, my back rounded before the bar left the ground. My hunch is that this is happening to you when the weight is heavier, and it might be causing your lower back pain. The way I fixed this was by thinking about bending at the knees to bring my shins to the bar. Your hips are supposed to drop a bit during step #3, but only to a certain degree, so just bend your knees until your shins touch the bar. At that point, keep your hips where they are and proceed to step #4.
Hope this helps. If a SS coach or more experienced trainee posts and thinks they know what's happening, please do feel free to ignore what I've said. Good luck!