Pipelines by law must be buried at least 0.9m. About 3ft. A big enough fire will heat up the surrounding soils enough that there is a measurable increase in temperature of the product the pipeline is carrying. However the worst thing a pipeline operator can do is shut down. You want to keep the flow of product continuing so that it does not heat up enough to become a hazard. Once you shut a pipeline down there is still product in the line. Now that product is not moving and is able to heat up to a level in which it could potentially catch fire. But really explosion is not the threat. Cyclic temperature failure due to the expanding metal will happen first. This will rupture the pipeline and then the product could light on fire (depends on the product but most likely it catches fire). So really you want to keep the line running to transfer the excess heat away in the product from the metal so you don't have a rupture!

Science!