The answer to 'why is my sink overflowing' is that there is likely a clog or blockage in the sink drain or the drain pipe. This can cause water to back up and overflow the sink. Common causes of sink overflows include hair, food particles, soap scum, and other debris accumulating in the drain over time.
The main symptoms of a sink overflow are water spilling out of the sink and potentially leaking onto the countertop or floor. You may also notice the sink draining more slowly than normal. To fix the issue, you'll need to clear the clog in the drain. This can often be done with a plunger, a drain snake, or a chemical drain cleaner. If the clog is deeper in the pipes, you may need to disassemble the sink or call a professional plumber.