It sounds like boredom and attention-seeking behavior. Puppies lack impulse control, there's nothing going on up there keeping them from going, "mom's gone so maybe they'll play with me! Hey, hey, hey pay attention to me!" They tend to ask for attention with their mouths. It's not a stress response, but one of an excited puppy who lacks other outlets.
What are you guys doing for training/exercise? Now is the prime time to start teaching manners and impulse control.