Maybe they too long for him or maybe he just simply bored and chewing on them for something to do. Dogs chew to also relieve stress, but from my own experiences with my own dogs over the years...something like that out of the blue is generally just being bored i think. Mine have all done it at one point, but stop just about as suddenly as they started. If he keeps doing it for a few days, yeah, maybe take him to the vet see whats up.