What may have caused a wall collapse in my cement septic tank (only 12 yrs. old)?

The site developed a sinkhole, and on further excavation, a contractor revealed the back wall had totally collapsed into the tank. It broke at a horizontal seam and extended the entire length of the back wall. When excavated, the top of the tank came out in 1 piece, but the bottom came out in multiple pieces as the rest of the tank broke up.