So is walking and almost anything else we do with legs, since that is the only real motion they ever do: back and forth. Walking would at least be the correct speed and alternating legs, but still be a terrible name for that leg swinging.
Not sure if it's culturally-specific but I think it's basically intended as a (likely involuntary) body language display showing interest and excitement. She is so engrossed in the explanation of Naruto (or the individual giving the explanation) that she is not paying attention to what her feet are doing. And what they are doing is acting as an outlet for her excitement.