I'm wondering of your glutes, hamstrings and adductors are up to par with your quads. Squatting deep powerlifting style calls on those muscles to drive you up from the hole. You're a little shaky at the bottom of your ascent, but smooth at the top.
Or it could be that you're still getting used to the movement
EDIT: From Crossfit
One of the common faults we tend to see during a squat, especially as the weight increases is the knees caving in. Typically, when the knees cave in during a squat it’s an indication of weak adductors. Your adductors are comprised of four separate muscles: adductor magnus, adductor brevis, adductor longus, and gracilis. These muscle attach up by your pubic bone and come down the inside of your thigh, attaching at the knee. The adductors are responsible for knee stability, especially during a squat. Weak adductors are potentially dangerous because the caved-in position of the knee results in unnatural forces at the knee joint. Bill Starr has a great article in the CF Journal about bringing your adductor strength up to par. Below is a list of exercises you can do to strengthen your adductors:
•Wide stance squats w/ toes pointed forward
•Front Squats (as low as you can go)
Check out the article by Bill Star in the CF Journal titled “Keeping Your Adductors Strong” for the comprehensive list of ways to strengthen your adductors. Don’t ignore this fault! Strong adductors will help you move more weight and keep you safe!