Air Conditioners leak water for a few reasons. It is possible that the air flow over the indoor evaporator coil is restricted due to either a dirty coil or a dirty air filter. In either case this will cause the evaporator coil to ice up and leak water over the edge of the condensate drain pan.
The most common reason for the air conditioner to leak water is that the condensate drain line becomes clogged causing the drain pan to overflow. During the warmer months of the year when the air conditioning unit is working hard there is a lot of moisture being pulled out of the air in the home and as this water drips down the evaporator coil and into the drain pan there is algae and other debris that form and run through the drain line. Eventually, if not maintained, this algae and debris clogs the line, forcing the water to overflow the pan or trigger a water detection device (float switch) that is connected to the drain that will shut the air conditioner off before it overflows.