For what it's worth, I would like to add my experience and theory to this.
I have had two female veileds and both showed the same behaviour. As they finished laying late at night, I left the laying bin in the viv intending to remove it the following day. Both of them would go back down the following morning and continue to shift the dirt. In the wild they would choose a suitable site, lay the eggs and leave. When they are in a viv they are obviously still in close proximity to the nest site. It was my thoughts that they could actually still smell the eggs, and therefore feel that they hadn't been 'hidden' well enough thus them carrying on with the dirt shifting. With both of them, once I had removed the laying bin, they didn't return to the bottom of the viv. Anyhow, that is just my 'take' on things.
