The answer to 'why is my drain trap rusty water' is that a rusty drain trap is usually caused by corrosion over time. The drain trap is the curved section of pipe under your sink or bathtub, and it can become corroded and start to leak rusty water if it's made of galvanized steel or cast iron. This is a common issue in older plumbing systems.
Some common symptoms of a rusty drain trap include discolored water coming from the drain, a foul odor, and water leaking around the trap. In some cases, the trap may even become completely blocked with rust and debris.