Rabbits burrowing.....?

We have recently aquired some chickesns, so we thought we'd let our rabbits out into the enclosure with them, as it's rather spacious. However, within munutes, our female rabbit started to dig under the fence. We've put rocks around the edge, but is there anything else we can do to deter it? also, is the rabbit trying to escape, or just making a nest? (it has had babies before)