1) Roots from trees may follow the moisture in the ground and find their way into cracked or leaking joints in the pipe.
2) Joints may fail over time and cause the inside of the pipe to lose flow area. Debris such as rocks or paper towels may cause a blockage.
3) Grease from cooking oils or soaps may settle in poorly sloped sewer laterals or behind failed joints.
4) There may be a deterioration of the pipe integrity due to age or poor backfill materials used during construction.