What causes a sewer line to collapse is rarely one thing. Old cast iron, clay or Orangeburg pipe corrodes and cracks with age; tree roots widen those cracks; ground shifts as expansive North Texas clay swells and dries; heavy loads pass overhead; poor original installation leaves weak joints; and low spots pool water that eats the pipe from inside.
Material and age set the baseline. Cast iron and clay pipe laid before the 1980s corrodes, scales and grows brittle, and Orangeburg, a tar-paper pipe used in some mid-century homes, deforms and flattens on its own. Once the wall is thin, tree roots that found a hairline crack or loose joint pry it wider as they thicken, and a hard blockage or jet of water can finish the break.
Pressure from outside does the rest. North Texas sits on expansive clay that swells in wet months and shrinks in drought, and that movement shifts pipe sections and pulls joints apart. Heavy trucks and traffic passing over a shallow line add load from above; an Ohio State University engineering study found weak concrete and repeated heavy loads are leading reasons buried pipe cracks. Poor original installation, with wrong bedding or a joint never seated, leaves a weak point that gives way years later.
Low spots matter too. Where a line sags into a belly it holds standing water and waste, and that pooling corrodes the pipe and loosens joints until a section caves in. A sewer camera inspection is the only way to confirm a collapse and tell it apart from a simple clog. Once a pipe has actually collapsed it usually cannot be lined, because a liner needs a mostly intact host pipe to cure against; that damage is repaired by excavating the section or by pipe bursting.