The answer to 'why is my backflow preventer constantly running' is that the backflow preventer is likely malfunctioning. A constantly running backflow preventer is often caused by a problem with the internal components, such as a stuck or worn-out check valve. This can allow water to continuously flow through the backflow preventer, leading to the constant running.
Some common symptoms of a malfunctioning backflow preventer include the sound of running water, low water pressure, and high water bills. In some cases, the backflow preventer may need to be repaired or replaced to stop the constant running. If the issue persists, it's best to call a professional plumber to diagnose and fix the problem.