The answer to 'why is my air handler leaking water' is that there are several common causes, including a clogged drain line, a problem with the condensate pump, or a refrigerant leak. The air handler is responsible for circulating cool air through your home, and as it does so, it collects condensation that needs to be properly drained away. If the drain line becomes clogged or the condensate pump fails, that water can start leaking out of the air handler.
Another potential cause is a refrigerant leak, which can cause the air handler's evaporator coil to freeze up and then thaw, leading to a leak. Symptoms of an air handler leak include water pooling around the unit, water stains on the ceiling or walls, and high humidity levels in the home.