The answer to 'why is my supply line overflowing' is that the supply line is likely clogged or the water pressure is too high. A clogged supply line can cause water to back up and overflow, while excessively high water pressure can also force water out of the supply line. Common causes include mineral buildup, a blockage in the line, or a problem with the water pressure regulator.
If the supply line is overflowing, you'll likely notice water leaking or dripping from the line. This can lead to water damage if not addressed promptly. To fix the issue, you may need to unclog the supply line, adjust the water pressure, or replace the line entirely. For more complex problems, it's best to call a professional plumber who can properly diagnose and repair the issue.