Most collapses we attend are in clay drainage laid before the 1960s where joints were sealed with mortar that has long since failed. Once a joint opens, soil washes in, the bedding under the pipe is lost, and the unsupported section eventually drops.
New collapses in modern pipework almost always have a cause: a driveway laid over a shallow run, a heavy skip or delivery vehicle parked over an unprotected pipe, or an extension built across a drain without a lintel or diversion.
The warning signs come months before the failure — recurring blockages, slow flow, subsidence over the pipe line. A CCTV survey at that stage usually means a liner rather than a dig.