I have experienced this and found cutworms just below the surface of the soil. This is from Wikipedia:
Cutworms are moth larvae that hide under litter or soil during the day, coming out in the dark to feed on plants. A larva typically attacks the first part of the plant it encounters, namely the stem, often of a seedling, and consequently cuts it down; hence the name cutworm. Cutworms are not worms, biologically speaking, but caterpillars
Cutworm larvae vary in their feeding behaviour; some remain with the plant they cut down and feed on it, while others often move on after eating a small amount from a felled seedling; such a wasteful mode of feeding results in disproportionate damage to crops.
Cutworms are usually green, brown, grey, or yellow soft-bodied caterpillars, often with longitudinal stripes, up to 1 inch (2.5 centimeters) in length