The answer to 'why is my evaporator coil leaking water' is that the evaporator coil is likely clogged or the drain line is blocked, causing the condensation to overflow. This can happen due to a buildup of dust, debris, or algae in the coil or drain line, or from a problem with the condensate pump. Leaking water can also be a sign of a refrigerant leak or other issue with the HVAC system.
If the leak is minor, you may be able to clear the drain line or clean the coil yourself. However, if the leak is substantial or you're unsure of the cause, it's best to call in a professional HVAC technician to diagnose and repair the issue. They can properly inspect the system, identify the root cause, and make the necessary repairs to stop the leakage and prevent further damage.