Sounds like someone needs some more squat training. This is by no means a bad thing though, and I think working on your squat form should be a journey, not a destination. That sounds corney.. but anyways..
Start doing some ass-to-the-grass pistols and lunges. Once you have those down, work in some front squats (a2tg), and then try overheads. Once I made that progression, I noticed my back squat was much more stable and comfortable... and I was able to go deeper than ever with a respectable amount of weight. Hope that helps!
I loves me some squaaats, can't wait until tomorrow.