The answer to 'why is my shut-off valve overflowing' is that the shut-off valve is likely malfunctioning or stuck in the open position, causing water to continuously flow through the valve. This can happen due to wear and tear, mineral buildup, or a problem with the valve mechanism.
Some common signs of an overflowing shut-off valve include water leaking around the valve, the valve handle being difficult to turn, or the valve not fully shutting off the water supply. If left unaddressed, this issue can lead to water damage and high utility bills. In most cases, you'll need to replace the faulty shut-off valve to resolve the problem.