when i do that its because im looking down, which causes my head to tilt down a bit, which causes my entire back to look down,,,
try focusing towards the cieling and lifting ur head back so ur back stays stright, and pushing more with your heels and the back of your body than rocking forward and pushing with your toes
mainly the looking up thing though, - if your looking up, (and keeping your back straight)- you will fail in the squat with your legs, before your back falls forward and down