Sags are caused by failure of the pipe bedding on the bottom of the trench in which your sewer pipe was placed. This type of failure causes a section of the pipe to drop below the proper grade. Water remains trapped in the sag and solids suspended in the water tend to settle in the sag area.
Eventually, the collection of debris in the sag will cause a restriction in the pipe and a blockage may result. Sags can only be fixed by removing the affected area of pipe and regrading the trench bottom.