The answer to 'why is my pressure regulator overflowing' is that the pressure regulator is malfunctioning and allowing too much water pressure to pass through. This can be caused by a buildup of debris, a worn-out diaphragm, or a faulty spring inside the regulator. Symptoms include water leaking from the regulator, high water pressure throughout the home, and the regulator making a hissing or whistling sound.
If the pressure regulator is overflowing, you'll want to first try adjusting the regulator by turning the adjustment screw. If that doesn't work, the regulator likely needs to be replaced. This is generally a job best left to a professional plumber, as they have the tools and expertise to properly diagnose and fix the issue.