The answer to 'why is my sump pump rusty water' is that your sump pump may have a corroded or rusted interior, causing it to discharge rusty or discolored water. This is a common issue with older sump pumps or those made with lower-quality materials.
Some common causes include mineral buildup, exposure to moisture, and general wear and tear over time. Symptoms include discolored water, reduced pump performance, and potential leaks. To fix this, you may need to replace the sump pump or have a professional inspect and clean the system.