It's a habit, but not a disorder. It's like an addiction, only you don't pawn your mother's jewelery to pop your knuckles or lock the door & newspaper the windows so no one sees you getting your jaw-cracking fix. The only problem I know it causes is: Since cracking joints releases little air bubbles in the hinge, the more you do it, the faster air is going to return & create soreness/tension. That, in turn, leads to more cracking & it kind of turns into a vicious cycle. First it's a silly habit, then you literally NEED to do it to avoid stiffness.