My Rat Terrier used to do this. Looking back i now realize her nails were probably a tad too long.

Other dogs plain simply retain the instinct to give their own manicures as already pointed out in this thread.

I sure wish King would have. His nails were SO VERY BLACK... I was always afraid of cutting the quick so they were always a little too long. *sigh*

I'm thinking that with a small population of canine nail biters that it could be a nutrient deficiency that they're trying to supplement (some dogs become stone chewers or poo munchers. I don't see where nail biting which their nails are made of bone is such a far stretch of thought in this.)