It's also a little lack of understanding of how the squat movement works. For example a low bar squat you sit back and push knees out as you descend, spreading the floor to create torque. You will hit a point where your arse will be back, knees out and not too far forward and back flat, at an angle where the bar is over your feet. The issue comes when people continue past this point by moving in a way which is bad, like going too far past parallel on a low bar squat, which requires your body to get the range from somewhere.
The aim of a squat is to go as low as the movement/your flexibility allows, while keeping the form.
It really helps to think about the physics and mechanics of your body, and question what happens at certain points of the movement. Look in the mirror side on and play with the squat, understand how everything comes together.
