it could be many things...
what size is your tank? is it possible he is stressed because he has grown and now it is too small?
any new tankmates since you got him, possibly bullying or outcompeting for food?
have you checked water parameters lately? (pH, nitrate, nitrite, ammonia)
do you see any sign of disease - spots, torn fins, slime, cloudy eyes?
any possibility the tank was poisoned - cleaning products, forgetting to treat water, etc.?
usually healthy, unstressed tangs eat like pigs. moving rocks around shouldn't do anything, unless all cover was taken away.