It might be nothing and she's just doing it because she can.
If it's a real tic it can be medically related and often times shows itself if taking a new medicine, a new stress, or is nervous, tense, shyness or in a new environment that is making her uncomfortable. If so she might not be able to stop it, easily and she might not even realize she's doing it.
You might want to get her a stress ball (stretchy material so she can use if needs to figet)
Are you stressed out or is DH? Was there an argument or maybe she overheard a conversation that bothered her??? (I'm not accusing you, just brainstorming)
I would make note of when it's happening - during homework? Before you ask her to do a task? Before bed?
Keep an eye on her and see how it goes.