Why does my dog bite his nails?

From the mail bag: My dog bites his rear toenails - not obsessively, but every once in awhile.  I don't mind because I don't have to clip them and I even used to do the same thing (bite my nails that is, not his).

Anyway, I'm just wondering if he could be doing any damage to his toes or feet that I'm not aware of.  It doesn't appear to be allergies, because he just bites the nails.

Have you ever heard or seen this?