I was wondering the same thing. But figured I missed something, just like I missed that there was another reply.

The reason for the backfilling is simple.....The pipe segments will be laid in by another crew...All the road crossing, stream crossings, etc will be done first. As is the rock trenching. As the pipe crew starts laying in the big 72" or bigger pipe, they will excavate the soft soil/dirt backfill is short sections. This allows a very expensive rock trenching operation to be complete in a short time and have trench that can be dug out as the line proceeds. Many times large projects use this method when the actual pipe laying might be weeks or months behind the trenching. Those trenchers don't sit very much. It's all production when they start if possible.

i thought that might be the case.