The answer to 'what causes drain' is that there are several common reasons drains can become clogged or backed up. The most frequent causes include hair, soap scum, and other debris building up in the pipes, tree roots growing into the drainage system, and improper disposal of fats, oils, and grease. These materials can accumulate and restrict water flow, leading to slow or completely blocked drains.
Proper maintenance and prompt attention to any drainage issues can help prevent more serious problems down the line. If you're experiencing persistent or severe drain clogs, it's best to contact a professional plumber who can properly diagnose and address the root cause.