Agree with Mark, slight misalinement or an instability/imbalance causing a "tracking" issue. These are normally related to the around the core, Glut (bum)tightness/inflexibility/under activation, tight IT band and incorrect quad activation.
If you can, try standing on the edge of a box, in front of a mirror and do a single leg squat, letting the leg thats off the box hang down the side with the working leg on the edge and straight. You have to activate your core and keep you knee in alignment, normally tracking over the 2nd toe but not past the toe (more of a sit down movement). You shouldn't have any inward (or real outward) movement, it can be hard to spot & may be a little complex try 10 - 20 each leg, any differences in feel or look, you backside should be a little "achey" from working.