The answer to 'backflow preventer overflowing' is that a backflow preventer that is overflowing is typically caused by a blockage or malfunction in the device. This can happen when debris or sediment builds up in the backflow preventer, preventing it from properly regulating the flow of water and causing it to overflow. Other potential causes include a faulty seal, a broken spring, or a problem with the valve mechanism.
If your backflow preventer is overflowing, it's important to address the issue promptly to prevent water damage and potential health hazards. In some cases, you may be able to resolve the problem yourself by cleaning or replacing the backflow preventer. However, if the issue is more complex or you're unsure of the cause, it's best to call in a professional plumber to diagnose and fix the problem.