A small margin for error in the manufacturing process
PVC pipes are manufactured by melting and extrusion of PVC powder. This chemical process, like most chemical processes, is vulnerable at many points along the timeline of manufacturing. If the extrusion process is not operated at the proper temperature the pipe is weak and can become brittle.
A weak and brittle pipe is not immediately noticeable, and can be installed under the assumption it can handle the load of a healthy pipe. However, a weak pipe put under high residual stress will begin to rupture over time and can cause catastrophic damage once a full rupture takes place.
The image above shows how high residual stress on a weak pipe can cause what is referred to as a creep rupture.