by Why does my washing machine drain overflow the standpipe?
Short Answer: Wastewater may be leaving the standpipe more slowly than the washer is discharging it.
Detailed Answer: Restrictions in the branch line, main sewer line, septic outlet filter, pump system, or drain field can reduce drainage capacity. Washing machines often expose septic problems because they discharge water rapidly.
Related Questions: Can washing machines overload septic systems? Why does laundry trigger backups? What are signs of a clogged septic outlet filter?