Armyworms or Cutworms do not burrow into the soil and dig tunnels bringing excess dirt to the surface to cause mounds. They reside in the thatchy layer of the turf in silky tunnels. My guess is earthworms as well, and they have a lot of beneficial properties for the turf (except for the mounds). Perhaps just fertilizing would thicken the turf up, and a little drier weather pattern might make the mounds not such a nuisance. Other "burrowers" might be mole crickets, I think someone mentioned an oriental beetle, I have also seen wasps burrow into dens and leave small mounds in some parts of the country.