Snapping or popping with a change in position is often caused by the tendon moving over the bone underneath it. Flexibility may have something to do with this.
Tendons move more freely in flexible people. Those with tight muscles and tendons have fewer (or no) episodes of popping because the tendon doesn't slide as much. It's more tightly bound to the connective tissue above and bone underneath.
Joint popping may occur for a variety of reasons. In some cases there isn't enough natural lubricant inside the joint to make joint motion smooth and noise-free. Muscle tension and spasm can also put stress on the joint forcing the natural lubricant to thin and changing the natural alignment. The result is the snapping, popping, or crackling often heard.
Cracking or popping the knuckles often produces a single pop that can't be done again for awhile. Pulling on the joint or distracting it changes the pressure in the joint. With distraction gas in the joint fluid forms bubbles making it easier for the surfaces to move apart making a popping sound.