Is the one that burries himself captive born? If not he may be trying to aestivate a bit, especially if he was wild caught at some point. Now in the wild I have observed that some spotted aestivate more than others. I have found that females are harder to find in the heat of the summer. However, males are out basking still and seen much more frequently. Another possiblility is maybe the cage is too crowded and he/she is simply trying to get away from the rest. Is it a hatchling or adult?