The answer to 'why is my drain rusty water' is that your drain pipe or plumbing system is likely corroding, causing rust to contaminate the water flow. This is a common issue with older metal pipes, especially cast iron or galvanized steel drains. Rust can build up over time, flaking off into the water and discoloring it.
Some other common causes of rusty drain water include mineral deposits, sediment buildup, and corrosion from acidic water. This problem requires prompt attention, as continued use of rusty drains can lead to more serious plumbing issues and potential health hazards from contaminated water.