im guessing my abs are weak, along with my hamstrings. My hips rise first maybe one out of every 5-6 squats (like it did today on rep 5 of 5 with 215, and rep 3 of 3 with 325... other wise its getting better.
Sometimes if you place the bar too low on your back you will lean formward to make up for the of balance center of gravoy, hence I do what is called a high bar squat where the weight is usually resting on the base of my traps and top of delt. I worked on this for months and got my form down perfect and never had a problem GM the weihgt.
I have the same problem, but in my case, I've done so much back work that I'm using it to compensate for weak hips, hamstrings, and glutes. When the weight gets heavy, it's just easier for me to lean forward, and force the weight up that way.
Flexibility in the hamstrings, glutes and hip flexors is very important too. It is very difficult to sit back and not bend excessively at the waist with short hamstrings.