Drain

• A clog in the drain line can cause water to back up into the washing machine after the machine drains and shuts off. Depending on the size of the obstruction and where it's located in the drain line, water might seep out of the standpipe behind the washer and create a puddle below the machine. Check the standpipe to see if it's wet or overflowing with water. If water fills the standpipe after the washer is turned off, contact a plumber to clear the obstruction.
  
  

Pump

• A pump will leak water if it starts to fail. When a pump works properly, it pushes water out of the washtub and into the drain to empty the tub. On most washing machine models, the pump is near the bottom of the washer, which is why, when it leaks, you may see wet spots below the machine. Contact a washing machine specialist to confirm that the pump is operating right, and have it repaired if a problem is found.
  
  

Water Inlet Valve

• The water inlet valve opens to fill a washtub, and closes when the washtub is finished filling. However, if the valve doesn't close completely, water will continue to flow into the tub even when the washing machine isn't on. When the valve won't close, it's possible that you may find water dripping beneath the washer as well as find standing water in the washtub. Shut off the washer's main water supply valves and contact a washing machine specialist to service the washing machine.
  
  

Water Supply Hoses

• Water will trickle out of a worn water supply hose and splatter on the floor behind the washing machine where the hoses are connected. Inspect each of the washing machine's two water supply hoses for imperfections. If one or both of the hoses are twisted, cracked or showing a bulge, replace them. The Repair Clinic website notes that the typical washer's water supply hoses last about five years before hoses should be swapped out for new ones.