Could be voles, or any number of animals looking for a meal - grubs, worms, buried nuts, whatever. You can try to reduce the grub population.

GrubEx is quite targeted at grubs, but I'm not comfortable using it because my property is near a municipal well. I use some products but only if I know they have low soil mobility or degrade soon after contact with soil/sunlight/water.

For grubs there's also milky spore and beneficial nematodes. I tried the latter and I'm not sure it had any effect. Ideally they will carry on doing their thing for some time, so I'll see if my yard is dug up come spring.