The answer to 'what causes backflow preventer' is that a backflow preventer is a mechanical device installed in a plumbing system to prevent the backflow of contaminated water into the clean water supply. Backflow can occur when there is a sudden drop in water pressure, which can cause water to flow in the opposite direction and potentially draw in harmful substances like sewage or chemicals.
Backflow preventers are required by building codes in many areas to protect the public water supply. They are typically installed at the point where the main water line enters a home or building, and they help ensure that any contaminated water cannot flow back into the municipal water system.