As far as I know, there are two major factors, fatigue loading and low temperatures.
Pipes buried below roads experience fatigue loading from traffic, surprisingly enough. Literally cars driving over the road surface imparts a load onto the pipes, even if it's a very small load. However, this happens many times every day and pipes stay in the ground for long periods of time. This means they experience millions of fatigue cycle loads over decades (50+ years isn't unusual for gas pipelines, at least).
Add in thermal cycling thanks to the weather and metallic pipes (and plastic, to a lesser extent) will become brittle under low temperatures.
You can see where this is going.
Corrosion caused by water is yet another factor etc etc etc and basically its all a surprisingly hostile environment for something which has to last so long.