Is there any pattern ? Is there any moisture at all ? Could it be your sewer tiles from the house to the main sewer are broken ?
I certainly hope not. They are widely scattered so that would seem to suggest against it would it not? Come to think of it, a neighbor down the street(maybe 10 houses down)has been digging up his front yard, seeking the root of some problem I've surmised, for months now. Maybe I should have a talk with him.