Over time sewer pipes can break. Whether is be caused by time, material durability, tree roots, or environmental changes pipes cannot last forever. When they do break or become cracked it allows for stormwater to enter the pipes causing a larger problem for the City.
Sewage back-ups in properties can be more likely if the pipes are not functioning as intended for. Tree roots have an opportunity to enter the sewer pipes when there is a crack and as the tree grows, those roots expand and can potentially fill the pipe entirely.
Pipes of any material (clay, PVC, iron, etc.) are all susceptible to aging and damage.