I have numerous comments
Don't walk back three feet from the rack. If for whatever reason you need to rack it mid set, you probably want it closer to you than it is now.
You need to take a second or two in between reps to to get a new breath and set your back and get tight. Stand tall, breath, squeeze, and begin the descent.
This will help with your lumbar because as it stands you don't have the time to get set up properly. (Though I'm not noticing a lot of flexion in these videos)
If you watch your knees during the second set you can see some knee slide at the bottom. It looks like your shoot your hips back too much at the start of the lift which causes youto be forward of the mid foot as you get to the bottom. Then your knees slide to get you back over midfoot. If you feel like the weight is getting on yours toes right before you get into the bottom this explains why. When descending, don't think just hips back, think hips down too. This will help you post your knees. Also, getting tighter before a rep will help with wobbly knees.
The weight moved fast though.