There are many possible factors that can contribute to patellar instability. Sometimes it's a simple matter of anatomy. The patella (knee cap) sits over the bottom end of the femur. There's actually an outward curve in the patella that matches a groove in the femur.
Anything that changes the alignment of these two bones can cause the patella to sublux (move to one side or the other of the midline) or dislocate. If the patella dislocates, it moves completely out of its groove and over to the side of the leg.
There can also be an abnormal Q-angle contributing to dislocation. The Q-angle is the angle formed by the quadriceps muscle and the patellar tendon. This angle represents the force the muscle can place on the patella. An imbalance in muscle pull can displace the patella.
An excessive position of genu valgum also contributes to patellar dislocation. Genu valgum describes the position otherwise known as knock knees.
Surgeons have also reported during surgery for this condition that there is often a very tight retinaculum along the outside edge of the patella. The retinaculum is a band of connective tissue. Releasing this tissue is often the first and simplest step to overcoming the forces contributing to patellar dislocation.