They dig holes just for fun. Or out of instinct. Or spite because they can and they know it bothers some humans. We had rabbits when I was a kid, and used them to mow the lawn. They got a little square cage made of compost wire, and we had to move it every 2-3h as they had finished the task and were going too deep. Also, they tried to dig holes all the time, which was prevented by changing the cage location. They're hard to catch once they're out.