Flashing. Sometimes fish rub against the sand for no reasons. My JD has been doing it since it was a Juvenile. If you know your water parameter are good and there's no sign of external/internal parasites, it should be okay.
If it's occasional, its normal.
However if you notice them doing it more just before or after a partial water change you might want to double check your water parameters and also make sure you're not waiting too long between water changes. If your nitrate readings creep up above 20ppm between water changes then chances are you need to be doing them more often.
Look at the fish carefully and see if he has any small white spots on him. That would be a parasite, ick. Like everyone else said, if you have had him for a while and not added any new fish and you water parameters are okay they flash from time to time. How often does he do this?