It could be tendons that need stretching; but it could also be inherent anatomical differences. I have curved shins, which pushes everything from the knee up a few inches backward when I try to squat. It's physically impossible for me to do a heels-down squat without bending my back to lean forward between my knees.
If that's true, you probably need to spread your knees apart more, and bend at the hip joint more to achieve full squat depth. Rounding your back == bad.
