The answer to 'why is my water line rusty water' is that your water line is likely corroded and releasing rust particles into the water. This is a common issue, especially in older homes with metal pipes. Symptoms include reddish-brown discoloration, metallic taste, and potentially reduced water pressure.
The main causes are corrosion and mineral buildup inside the pipes over time. This can happen due to the chemical composition of the water, age of the plumbing, and other factors. While it's not an immediate health hazard, rusty water can stain fixtures and laundry and has an unpleasant taste and odor.