I have never seen Trapp or Jasper do it,but I have seen Earnhardt do it many times.I look at his paws often,as he is not really cooperative about manicures.
Interesting observation really.
When he is doing it,he does not seem to be trying to chew his nail off or make it shorter.I just thought it was part of his cleaning routine.When he does it,I always make sure to check the paw to see if there is something funky going on,and there never is~