The hip flexors and lower back muscles both pull the pelvis into anterior tilt (think of standing with your butt pushed back and your lower back arched). As one descends into the squat, the more flexible the hammies are, the lower one can get before they lose that lower back arch. Once they get to the point where their hamstring flexibility is insufficient, their lower back will round. This assumes that they are keeping their chest up and sitting between their legs while squatting, etc...
To get better at overhead squats, I like sitting in the bottom position for a few seconds on each rep.
Also, like someone else mentioned, you don't wanna do sets of 12 in this exercise. Your wrists will like you!