The answer to 'what causes sewer line' is a variety of issues, including tree roots, pipe corrosion, pipe clogs, and ground shifting. Sewer lines can become clogged over time due to buildup of debris, grease, and other materials. Tree roots can also grow into and damage sewer pipes, causing cracks and blockages. Older metal pipes can corrode and deteriorate, leading to leaks and collapse. Ground shifting due to soil erosion or earthquakes can also damage sewer lines and cause them to break or become misaligned.
If you're experiencing sewer line problems, it's important to have a professional plumber inspect the issue and determine the root cause. They can then recommend the best solution, whether that's clearing a clog, repairing a damaged pipe, or replacing the entire sewer line.