An aside, not pig related, or is it?
Has anyone considered whether some ditches have no deliberate function but are merely incidental artefacts, created by the removal of material for constructing banks or mounds? You can see the phenomenon all the time on beaches, when kids are making sand castles.
In a world without wheels, getting material from the nearest place might have been important.
This is not to say that, having produced the ditch it was unwelcome (and not utilised as a pig track or appreciated for its aesthetic effect or whatever) but its possible it wasn't part of the original scheme aims and only the bank was of ritual importance.