Clay movement can leave a sewer line out of alignment.
River Oaks clay dries out and contracts, then takes on water and expands again. A buried sewer line depends on a steady slope and secure connections from one end to the other. When the soil moves around it, joints can separate, a section can shift, or part of the pipe can lose the grade needed to carry waste away.
A low spot, crack, or displaced joint catches paper and solids. Clearing a blockage may restore flow for a while, but material keeps collecting where the pipe is no longer smooth and properly aligned. Repair focuses on restoring that section of the line instead of repeatedly reopening the same restriction.
Older development also raises the chance that a sewer line includes older drain materials or has had isolated repairs over time. Those pieces do not always act like one continuous modern pipe. Looking through an accessible cleanout helps distinguish roots or buildup from separated piping, a localized obstruction, or a section that has dropped out of grade.