To be fair, your lumbar is rounded on some of the reps that I was paying attention to, anyway (your own fault for posting some of those gifs).
Get your core really tight, chest up, butt out and push your hips through...
And then, on the descent, give your knees a 5% bend, and then sink your hips back, keeping your core tight (as on the way up) all the time.
You doing Low or High bar squats? Only asking as the bar looks like its sitting high up on your back.