Bellies are caused by loss of support under the pipe — washed-out bedding, ground movement, poor compaction during original installation, or subsidence near trees.
On CCTV a belly shows as standing water that does not drain away between camera passes. It is one of the most commonly missed defects because it is not a crack or a break.
A shallow belly can sometimes be managed with regular jetting; a significant one needs excavation and re-lay to restore the fall, as lining will simply follow the existing shape.