1. You don't know proper technique.
2. You don't care or focus more on just lifting the weight.
3. You have mobility or strength limitations in one or more joints.
4. Two or all of the above.
If you don't care, then you wouldn't even be reading this post anymore. If you do care, then I can teach you proper technique and I can help you develop the mobility and/or strength you need to squat properly.
A. The Set-Up: Place your feet shoulder width apart, keep pressure on the outside of your heels, point your toes slightly outward (10-15 degrees), retract your shoulder blades, and keep your chest up.
B. The Descent: Keeping your spine tall, push your hips back, open up your knees, and slowly squat down.
C. In The Hole: This is the end of the range of motion. Spine and head should be neutral (tall). Knees should stay in-line with toes and not buckling inward. Pelvis should not tuck under. Shoulder blades should stay back, as the chest should stay upward.
Finish by driving with the hips, through the heels. Torso should have minimal movement. Hips and knees should have ~90 degrees of motion, while the ankle should have ~45 degrees of motion.
If you can't squat like this, work on your weakness (technique, mobility, stability) BEFORE significantly increasing the weight! That way your lower back and knees will be happy, and you can steadily build your bodies' strength and function.