One thing I would add (as Lucas mentioned) you appear to be in an over extended position to begin the squat, which really accentuates the buttwink.  Try setting your pelvis in a more neutral position (ie not so hyperlordotic) prior to starting the squat and create that torque David mentioned.  You can achieve this more neutral position by slightly tucking your tail bone under and squeezing your glutes.  Break from the hips and then maintain that pelvic neutral throughout the movement.  That way you don’t get the huge swing from extension to flexion and back to extension, because at the end of the day it is that what will lead to injury.

I’ve spent many nights staring at the ceiling, contemplating the mechanics of a ‘simple’ air squat!