Just Google "tractor gas pipeline explosion" and you'll notice some very nasty pipeline/machinery contact events in the results.
Even just mowing alongside pipeline easements is exceptionally hazardous, and more than one tractor operator/farmer has died in a ball of flame when he hit a pipeline valve hidden in the grass.
Pipeline owners get very nervous when any heavy construction or farm machinery approaches their pipelines, so they like to be proactive, and head 'em off at the pass.