The answer to 'why is my sewer line rusty water' is that it is likely due to corrosion or rust buildup in the sewer line. Over time, the metal pipes used in sewer lines can start to rust, which can cause the water flowing through them to appear discolored or rusty.
Some common causes of rusty sewer water include old, outdated plumbing materials, high mineral content in the water, and chemical reactions between the water and the pipe materials. This issue is more common in older homes with aging infrastructure, but it can happen in newer homes as well.