The massive amount of trenching is due to a combination of factors:
Visible in the photo above is mostly the second, 1970s-1980s system, in white PVC.
As this was the first system removed, i attempted to remove the entire length in one piece, assuming that it might have a future, non-plumbing use. After a couple of pipe snaps and discussion with Siber Hussy, the remainder of the galvanized pipe was intentionally snapped at weak points to create trash-can-sized pieces.
Here can be seen part of the wackiness as the hose bib feed plunges right across the path of the east circuit.
Visible in the two close-up dirt photos above, and featured in the photo nearest this paragraph, is a remnant of the original system which terminated directly under the gate, around the middle of its width. The section pointing towards the red hummingbird feeder was part of the line going towards the back of the property. This entire section was disconnected at the tee (visible) when the 1970s-1980s system was installed.