The answer to 'why is my drain overflowing' is that the drain is likely clogged, preventing water from properly draining. This can be caused by a buildup of hair, soap scum, food particles, or other debris in the pipes. The symptoms may include slow draining, gurgling sounds, or water backing up into the sink or tub.
In many cases, a clogged drain can be cleared using a plunger, a drain snake, or a chemical drain cleaner. However, if the clog is severe or the problem persists, it's best to call a professional plumber. They have the tools and expertise to properly diagnose and fix the issue, preventing further damage to your plumbing system.