Eating dirt is fairly common with veileds. I have never seen mine doing it however. It makes me believe that they are doing this for a reason.
A lack of nutrients/minerals in there diet. It is my thought that some animals will do what they can with resources available. The results are typically in examples like if the animal is in need of hydration it will eat leaves and my opinion if an animal lacking nutrient/minerals will eat dirt. Im not saying veileds are like humans in anyway but read a little bit on how kids will eat dirt when they are low on iron. There is actually many other similar stories where there is a deficiency and we hunt for sources unknowingly.
To solve this, a more rounded gut load and starting a multivitamin in the diet should help/.