The answer to 'why is my sink overflowing' is that the sink drain is likely clogged, preventing water from draining properly. This can cause the sink to fill up and overflow. Common causes of a clogged sink drain include hair, food debris, soap scum buildup, and foreign objects.
Symptoms of an overflowing sink include water pooling around the sink, a slow draining sink, or water backing up into the sink from the drain. To solve the problem, you'll need to unclog the drain. This may be a simple DIY fix, but if the clog is deep or difficult to reach, it's best to call a professional plumber.