Possible, but it still looks to me like her skirt actually comes down, not just moves in a swaying motion. The rest of her body stays to still, it really looks like the skirt moving on its own, not a result of other body movement. Just what I'm seeing.
I can understand what you are seeing. I tried to visualize what you described but it didn't work for me. To me, it seems very like she leaned back slightly thus causing the skirt line to lower an inch or two. You can confirm that by looking at the top line of her raised arm. At that same moment, the arm line also dropped by about the same amount.