If everything husbandry wise is good,there are no signs of a R.I. and you don't have mites. Then I wouldn't worry about.
MY bp too started doing this when he wasn't in shed. It wasn't excessive but it made me go "What the heck? What is going on?" but he quit. And he is quite alright :] And has been since he quit doing it a couple months ago. So as long as it isn't excessive and you've ruled out everything then don't worry about it. :]