Evening Hair - guessing an old critter hole, depending on what you have up there. Tunnels can be extensive and if the ditch either intercepted it or you just uncovered an old entrance or ventilation hole, you could lose a lot of water in it before it fills up.
Critter hole indeed seems to be the only reasonable explanation, unless perhaps it was seeping into a hobbit's old attic. ;~D
If I'd have known I had a drain leading out right there I wouldn't have needed a ditch!