If the installer of the underground piping did not use proper bedding or grillage to support the underside of the piping upon backfilling, this could easily create a belly in the pipe system.
More than just the belly itself, if PVC piping was used in this replacement, the strain on the piping itself and the fittings attaching the pipe will either eventually split or shear off due to the material not being used for its designed purpose.
I cannot stress enough that the liability lays on the plumber, not the individual backfilling because if the plumber had the work inspected, he or she knows very well that all piping installed underground must be supported and be surrounded by proper grillage if piping is not supported by virgin uncut earth.