Squat depth can be affected my many things. Most people initially blame a lack of hip mobility, but your ankles can greatly affect your range of motion and ability to maintain a safe technique. The picture shows Coach Garrett after performing some mobility drills on the right ankle only. As you can see, the right knee is able to track further over the foot. This will allow your hips to drop closer to your heels, rather than behind them. If your ankle can't flex forward, the weight will be behind your center and affect your balance. In order to put the weight over your midline, you'll flex at the waist, putting your spine in a vulnerable position. If you don't know any ankle mobility exercises, ask a coach at your next class!