shallow burrow in a raised vegetable garden


we have a small raised vegetable garden 1.5 metres high and I found a shallow burrow right in the middle. It was approx6 inches in diameter and 12 inches deep filled with dried grass. I cleaned it out and replaced the soil but have no idea what could have caused it and if its likely to return


Thank you for your inquiry. You may wish to refer to one of our previous post on Burrow holes in garden for some possible culprits.