The answer to 'why is my zone control system leaking water' is that the most common cause is a problem with the condensate drain line. The zone control system produces condensation as it cools the air, and this water needs to be drained properly. If the drain line becomes clogged or has a break, the water can back up and leak out of the zone control unit.
Other potential causes include a faulty zone damper, a problem with the zone control board, or an issue with the zone sensor. In some cases, the leak could also be due to a plumbing problem unrelated to the HVAC system. If you're unable to identify and fix the source of the leak, it's best to call in a professional HVAC technician to diagnose and repair the issue.