I also think that her throat should be checked out , sinus problems and tonsils may play a part here, I have many allergies this year I believe are environmentally caused and one is a scratchy throat I want to clear it a lot. good luck
I used to do it when i was her age, maybe younger and it was just a habit, I felt very weird if I didn't do it, Oh how my mom hated it! she would constantly point it out to me and others, so I completely stopped having family moments, I'd rather stay home alone than going out for ice cream with her, it stopped by 5th grade because I had so much to do in school, home and I would have died if I didn't go out and play, so I guess that little by little I forgot about it. ask her why she does it, but try not to make her feel awkward, try to see it as a baby sucking her thumb, it'll pass
Might take an ENT doc to answer that. I would be tempted to try an anti-histamine and see if it is allergy caused (given the season).