The answer to 'why is my furnace leaking water' is that a leaking furnace is usually caused by a problem with the condensate drain system. The furnace's heat exchanger produces condensation as it operates, and this water needs to be properly drained away. If the condensate drain becomes clogged or the drain line is broken, the water can back up and leak from the furnace.
Other common causes of a leaking furnace include a cracked heat exchanger, a problem with the condensate pump, or issues with the furnace's humidifier. Symptoms of a leaking furnace may include water pooling around the unit, moisture on the walls or floor, or a musty smell.