I'm going to guess that as the canopy gets taller and narrower, trapped pressure builds up inside the canopy, and begins to leak around the edges, "deflating" the canopy to a wide diameter - which then sweeps up more air per second, reinflating it to tall and and narrow - repeat as needed.
It is sort of the inverse of how "canopy-shaped" jellyfish propel themselves:
https://www.youtube.com/watch?v=rzNI1Yh0F5A