it's often hard to tell without doing some assessments - there's no one "the cause" - anything from shoes that stop the feet from moving all their joints, hips that don't know how to move without the pelvis, plain old weakness - any of these things can have an effect on a knee issue.
If you don't have a movement specialist near by, i do a lot of work with athletes (and their coaches) via web cam. Let's set up an assessment as said if you don't have a local expert.
Once the issues addressed (often movement related) it's largely a case of learning some drills that can be practiced to address the issue.
dr mc schraefel, phd cscs