Are you losing your neutral spine position at the start of your squats? This is one of the most common mistakes that I see people make when squatting. It's often (but not always) caused by a combination of 2 things. 1) Using a "hips back" squat pattern. 2) Not maintaining a strong core brace throughout the squat.
I've posted about both of these things recently here and here. Check these out for more info about a "hips back" squat pattern and how to brace your core correctly.
If someone is not bracing their core AND they think about sending their hips back at the start of their squat, they tend to anteriorly rotate their pelvis. This basically means that they arch their back (think of the "cow" portion of the cat-cow yoga pose). When they anteriorly rotate their pelvis, they lose a neutral spine and pelvic position and set themselves up for poor spine and pelvic positioning throughout the remainder of their squat.
You'll often see the person move out of this anteriorly rotated position and move into a posterior pelvic tilt (rounded back) as they move through their squat. In the video below, you can see that my low back begins to round when I reach parallel, and continues to round as I reach depth. At the bottom of my squat, I am in a posterior pelvic tilt or commonly called a "butt wink" position.
You'll also see that as I come out of the squat to standing, I overextend my low back and hips by squeezing my glutes and pushing my hips forward. This is another sign that I am not maintaining a core brace throughout the entirety of my squat. At the top of the squat, I prefer that athletes think about standing straight up and maintaining a neutral spine using a braced core. Over-exaggerating the glute squeeze, especially without coinciding bracing of the core, will send the hips too far forward and cause the low back to arch.
Why is maintaining a neutral spine important? 1) Improved performance! Maintaining a stacked spine and stiff trunk during your lifts can assist you in lifting heavier weight. 2) Less strain on your low back overtime.
If focusing on your core brace and your technique doesn't fix the problem, there's a chance that you have some tight muscles or a muscular imbalance that is preventing you from maintaining a neutral spine throughout your squats. Reach out to me or your trusted PT to get evaluated if you need more assistance with this.