Most sewer backups happen because the line is plugged with debris. Typical solids that build up in the pipe and cause backups are fats, oil, grease (FOG), dirt, hair, bones, sanitary products, paper towels, kitty litter, diapers, broken dishware, garbage, eggshells, coffee grounds, and concrete.
Tree roots can also cause backups. Tree roots are attracted to wastewater lines because of the waters' warm temperature and nutrients. Roots can infiltrate the pipe system and clog wastewater flow.