Why does the region behind my knees (or the popliteal region) sometimes feel bulging?

Benign Cyst. Swelling behind the knee is usually caused by a baker's or popliteal cyst. It is a fluid filled extension of the knee joint that pops out between the muscles and tendons in the back of the knee. If painful, they can be drained in the doctors office using ultrasound guidance. Rarely surgical removal is necessary.