Whenever i sit criss-crossed or on a chair withmy legs crossed my knees sometimes pop. i have to do a kick motion to fix it. its painful. any causes?

Sublaxation: it sound that you have a sublaxation of the patella the ligament that holding the patella are loose and trying to take it out of it's place wear a knee brace and see a knee specialist before it come completely out of place

Varies: There are several possibilities. A plica on the knee is one possibility. Don't hesitate to be seen for a specific diagnosis.