Well one thing is that on every rep you’ve already locked your knees before you even start to straighten your back, so you’re pretty much just relying on your spinal erectors to lock it out. Maybe this is a factor in why you’re feeling it so much in your lower back? It’s hard to say exactly why because heavy deads work your lower back quite a bit no matter how you do them.
Few other things I noticed:
You take waaaaaaaaaay too long to set up. Longer setup = energy leak. Set you foot position against the bar, bend down and set your hand position, pull the bar in tight to your legs, and pull right quick.
It looks like the bar is about an inch off your body in the video. That’s bad because it’s a mechanically weaker position. You will be pulling at your strongest when the bar is tight with your body and basically sliding up. That’s why we all have bruised up shins lol. Long socks are a blessing. Something I noticed that might help with this last point: you don’t seem to be pulling your shoulders down and back. If you squeeze your lats, upper back, and rear delts, you will pull your shoulders back.