The answer to 'why is my water softener rusty water' is that a rusty water softener is often caused by corrosion within the unit itself. This can happen over time as the internal components wear down and rust. Symptoms of a rusty water softener include reddish-brown water, stains on fixtures, and a metallic taste or smell.
The most common causes are improper installation, using the wrong type of salt, or simply age and normal wear and tear. In some cases, the water softener may need to be replaced if the internal corrosion is severe. However, in many cases, the issue can be resolved by cleaning the unit, replacing the resin, or making minor repairs. If you're not comfortable doing this yourself, it's best to call in a professional plumber to diagnose and fix the problem.