Ah...this should be instructive.
He's doing overhead squats but the principle is the same. Look at the sequence and see how the lower back rounds under at first and then he corrects it. Now he is not going anywhere near deep as he could go but you could imagine that if he were to go deeper that butt wink would just increase.
And click the "more info" on the right side. It's correct that motor control is a big key. But if you wanted to increase depth you have to increase mobilty in the hips and ankles AND stabiilty of the core and lower back as well as motor control.
Now when you consider back squats and the large amount of spinal compression that makes it all the more dangerous to have that lumbar rounding.