Why is my dog bitting his nails?

My dog has been chewing the heck out of his toenails. He chews them up so bad that they get mangled and splinter up pretty bad. When I catch him, I say NO!! and smack his nose and hold his pay so he cant get to them, but when I not home or when Im asleep he continues to bite them. he is just over a year old and he started doing this a few weeks ago. What can I do to help stop this behavior? And why is he doing it in the first place?