The usual cause would be an excessive amount of ice build up in the coil. The coil is located,
usually in the attic with the furnace. The reason it will build up ice is because the freon in the
copper lines has a leak, causing an imbalance in the weight of the freon will causes improper flow and ice.
Central air units naturally create water during the cooling process, especially during particularly humid weather. This water usually is piped outside. If leaking, piping may be clogged.