C.R.
I had have read that at this age it is very common for children to develop certain habits. Like chewing on the collar of their shirt, hair twirling and stuff like that. At this age they are really becoming aware of them selves in the world and how they are perceived by other peers. New pressure. Becoming more "thinky". Not sure just how to deal with a bad habit at this age but understanding child development at this age would be a good place to start :)
C.