Hmm it can mean a few things:
1. Temps are not high enough
2. Just for the fun of it, both genders love digging.
3. If it is a female and its in with a male, maybe gravid.
Thats about all I can think of at the moment, may I ask what your Temps are, if that beardie is housed with others, etc.
my male does it and he is in with another male so definatly not gravid. i have watched him and he diggs it and then sleeps in his digged up area or sometimes there is a meal worm under the sand that has excaped the bowl and he eats it.