my panther also does this, he started at about 4 months. I think he's really trying to get to the soil.
I posted asking the same thing (and have also done a number of searches since) there really doesn't seem to be a firm conclusion, some supsect that they maybe looking for some vitamin or mineral that is lacking in there diet and at lest one member (who is also a farly lage breeder) screens (so there is no sticks or large bits) his dirt and allows his cham to take a snack at will.
I still have mine covered with rocks myself, I've started alternating using reptivite and herptivite (for vitamins) and alternating repti-calcium and Miner-ALL O for calcium (hoping to provide a better balance of vitamins and minerals), he doesn't do it all the time but I still see him bite at a rock now and then (and it has nothing to do with hunger or lack of food as I'm still feeding him as much as he wants daily)