Thank you for asking your question on Just Answer. Often the main sewer or other parts of the drain can be clogged and cause this problem. Here is an example. Take a paper cup and put a small hole in the bottom. Now take another cup the same size and pour water into it. The water from that cup will never cause the water to over flow from the cup with the hole in it. If you pour water into the cup with the hole in it, from a larger cup, the water will overflow like the water backs up in the shower. The same thing could be happening to your plumbing causing the back up. As soon as the clogged drain gets filled up by the water being pumped out of your washer it will back up. The only other possibility would be the vent pipe from the plumbing being clogged. Something would have to be run down the vent pipe to verify that problem and remove the clog to fix it. The drain problem will most likely require a plumber. Most drain cleaners will do more harm than good.
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