Crows do that here too. Skunks do it looking for June bug grubs. My cat uses the mulch for litter, but covers the hole with a pile of mulch that reminds me of Close Encounters of the Third Kind…
Might not be what you have there, but one time last summer I I was on my knees doing some weeding under an apple tree that had wood mulch, and the weeds were starting to come in at the edges some. I got tired of being on my knees and sat down - 20 seconds later a snake came barreling out like it had been shot out of a cannon.