Bump to knees over toes and chiropractor. An X-ray of the hips will tell the story.

It's very common for people to have mismatched leg lengths. The shorter leg might be .5 -1cm smaller than the longer leg. The short leg takes a little more load and puts extra stress in the knee. Orthotic inserts or a heel lift in the shorter leg shoe fixes it. Some PT /weights after getting some correction solidifies it.