A 57-year-old patient complains about mobility of his metal-ceramic dental bridge supported by the 33, 37 teeth. The bridge has been in use for 9 months. Objectively: X-ray picture shows alveolar process atrophy by 2/3 in the area of the 33, and by 1/2 of root length in the area of the 37; there are pathological pockets, gingivitis. What is the cause of pathological mobility of supporting teeth?