Sewer lines can be damaged or blocked because of a number of issues. Tree roots are the number one source for collapsed or clogged sanitary sewer lines.
Tree roots often seek out sewer lines because they provide warmth, food and nutrients, which leads to sewer damage.
Warm air circulates in your sewer line, causing condensation to form and escape into the soil. The condensation attracts tree roots which find their way into tiny cracks, loose joints, and other openings in the pipe. This provides a constant supply of water for the roots to grow, and eventually leads to a blockage or total collapse of the sewer line.