You may have a problem with your vent system. A washing machine flows a lot of water fast, and if the vent system isn't right, could siphon out the traps in the house. Have someone listen carefully at a sink when the washer is draining to see if they can hear air or sucking sounds.
Now, if this is found to be the problem, whether it is a plugged line or bad design, you'd have to be there to tell.