When the lowest plumbing fixture in the house overflows when another fixture (like a clothes washing machine) is draining; most commonly the septic tank needs to be pumped, there is a break in the sewer pipe outside the house or the main drain is plugged somewhere. However, the washing machine line could be connected to close to the “suds rinse zone”, meaning the washer waste ties into the waste or soil line of another fixture too close downstream from the problem fixture. What is happening is the water is rushing by the suds at a high velocity, pushing ahead of the suds. Because the fixture is the closest place of relief, the suds will come up into the fixture, even a toilet. The code requires that a washing machine, kitchen sink, shower, and dishwasher line be connected at least 5′ downstream from any fixture branch. This could be just one of many possibilities for the bubbling and backup. We strongly recommend contacting a licensed plumber to help you diagnose your individual situation.