It is a self-soothing behavior that is instinctive in infants that have suffered trauma.
It also surfaces in adults who are subconsciously triggered to feel on coming anxiety.
I suffer from this and it's become extremely distressful, embarrassing and now amplifies my anxiety rather than calms it.
Hi. First of all, you have to verify if she is indeed aware that she's doing this. If she's not aware, then this can be a mannerism, or a subconscious act. Lack of focus or concentration can also lead some people to do these things. If there are no other symptoms present, then I believe this is nothing pathologic.
Regards and God bless.