Muscle atrophy occurs after prolonged disuse of your muscles. The jawbone requires a considerable amount of stimulation to grow, develop, and sustain itself. The only way it receives this stimulation is through the movement of the jaw while biting and chewing. In fact, the pressure of chewing helps strengthen the jawbone.
Tooth loss reduces the amount of stimulation that the jawbone receives, causing it to lose mass. Moreover, tooth loss also causes the alveolar bone (the ridge of bone that supports the teeth) to deteriorate. Once the jawbone loses its stimulation, your surrounding teeth begin to lose support, causing them to shift from their place.