It's a show of dominance, nothing more. Right now my son's beagle puppy (7 weeks old) is doing this with my daughter's 5 year old mixed breed terrier. An older dog will do this to a younger dog as if to tell them "OK, pup, you've crossed a line". There is really nothing more to it than that.
Young dogs will bite and nip in a bid for attention. It's perfectly normal and we are going through this here now. /sigh I know it will pass, but getting the kids to understand it has shown me that no matter how grown up they think they are, they really don't get being a kid.
The pup is pushing the boundaries, your dog is defining them.
Knitting to keep my sanity. It's working... kinda...
OTN: Dr Watson's Cable Crew Neck
OTN: Writer's Cardigan