Why do refrigerators leak water.?

Assuming that it's not a leak in the ice-maker or cold water dispenser... Water condenses inside a refrigerator and on the freezer coils. When that water is removed (self defrosting freezers) it drains down a tube and to a tray at the bottom of the refrigerator. The HOPE is that the water in the tray evaporates, but sometimes, especially if you live in a very humid area, the condensate will build up faster than it will evaporate from the tray.