The answer to 'why is my P-trap rusty water' is that a rusty P-trap is often caused by corrosion and mineral buildup over time. The P-trap is the curved pipe under your sink that holds water to prevent sewer gases from entering your home. When the metal in the P-trap starts to rust, it can discolor the water and lead to a rusty appearance.
Some common causes of a rusty P-trap include hard water, high mineral content in the water, and general wear and tear over the years. In some cases, a rusty P-trap can also indicate a more serious plumbing issue, such as a leak or damaged pipes. If the rust is severe or the P-trap is leaking, it's best to call a professional plumber to inspect and replace the P-trap.