The answer to 'why is my humidifier leaking water' is that there are several common causes, such as a clogged or damaged drain line, a faulty float switch, or mineral buildup. Leaks can occur if the humidifier's water supply line is loose or cracked, or if the humidifier pan is cracked or not seated properly.
If your humidifier is leaking, first check the drain line to make sure it's not clogged with debris. You can also try cleaning the mineral deposits in the humidifier pan. If those simple fixes don't work, the issue may be with the float switch or other internal components, which would likely require a professional HVAC technician to diagnose and repair.