by Why does my utility sink back up when the washer drains?
Short Answer: The shared drain line may be unable to carry the discharge volume.
Detailed Answer: A partially blocked branch line, main sewer line, septic outlet filter, pump problem, or drain field issue can cause washer discharge to back up into nearby fixtures connected to the same drainage path.
Related Questions: Can a clogged septic filter cause whole-house symptoms? Why do lower fixtures back up first? How do I diagnose laundry-related backups?