Because after squatting back so far and releasing the hip and lower oblique muscles, you must first raise the head to raise out of a deep squat. If while descending into a squat, the glutes go back first, then the head must move last. The opposite of this eccentric phase is an concentric contraction, or raising. It only stands to reason that the head must raise first, and the glutes will follow. We see many lifters get bowed over coming up from a squat because they push with their feet first instead of their head.
Static work overcome by dynamic action builds explosive strength best.
This is precisely what a box squat provides. In addition, you are always breaking parallel. The graphical representation looks like a wave, thus the name. The box allows one to max out and to determine your new strength level without the aid of contest gear wraps, straps, and a big psych. The contest max requires a large crease of adrenaline, thus causing psychological regression of the central nervous system.