In order to keep the bar tight to your back you should be pulling that bar down and pushing your elbows forward.
Going ATG gives you more momentum and that's where this 'pop' comes from. When it happens to me it's because I'm using my quads and pushing my knees back instead of pulling my hips forward. I think that is what another poster was trying to convey.
A squat should use close to equal quad/ham/hip in order to complete the movement.
If your focusing primarily on speed and explosiveness; then you'll be ok. But if your just shooting for a massive squat number, your going to hurt yourself 'poping' 400lbs on your back.
Good luck figuring that shit out man!