I noticed this last year on the last clutch of the season, and I'm also noticing it this year with my first two. The females seem to dig like mad at first with their front two legs. And then just before laying they start to dig with their hind legs. They alternate between the two hind legs, kicking out dirt or in this case substrate. I'm wondering if this shift from one leg to the other with the hind legs helps with the labor.