The answer to 'why is my toilet rusty water' is that the rusty water is likely caused by corroded plumbing components in your home's water system. This can happen when the metal pipes or fittings inside the toilet or connected to it start to deteriorate over time, releasing iron particles that discolor the water.
Some common causes of rusty toilet water include old galvanized steel or cast-iron pipes, a corroded tank or bowl, or even issues with the municipal water supply. Rusty water can also indicate a larger problem with your home's plumbing that may require professional diagnosis and repair.