Watch any wall-unit air conditioner and you'll notice a steady drip or even stream of water pouring from one corner. This water is the condensation that forms when humid air comes into contact with the cold heat exchanger. Your refrigerator doesn't have the benefit of draining onto open ground, which is why it has a drainage pan underneath. The water in the drainage pan evaporates away faster than water can condense on the coils, so it should never overflow.
Blocked Drainage Hole
By far the most common cause of water leakage involves the fridge's drainage hole, which can get clogged with ice and debris. Instead of draining through the hole, water pools around the inside of the refrigerator case and finds its way out through the nearest hole. This hole will more than likely be near the corners of the refrigerator housing where it bolts to the chassis underneath. Instead of going into the drain pan, water dribbles down the case and pools on the floor.
Correcting a blockage is fairly simple as most manufacturers design their products with periodic cleaning in mind. First, pull the lower cover off of the front of the unit and pull the drain pan out; there's a good chance the water inside will be rancid so hold your nose and throw it in some bleach water. Pull the fridge away from the wall and remove the rear cover; look below the radiator-like evaporator coil for a roughly 1/2- to 1-inch diameter hole. Poke a length of coat hanger wire into the hole to clear away any junk or ice blocking it.
Many refrigerators also have a drain hole inside their lower compartment, which can also get blocked and contribute to leakage. In some cases, water condensation inside the refrigerator can cause the drain pan to overflow. If your fridge has an automatic ice maker or water dispenser, check the connections along the water supply lines and filters. A refrigerator with a front-ward tilt can also be prone to leaks, primarily because water runs out of the front instead of through the drain holes.