I would look at how the video camera compensates for the lower light
level in the cloud. If it changes its scan/shutter rate it would
cause the apparent change in relative movement of
the propeller. Also these apparent flicker/stroboscopic patterns are
not always directly related, that is the prop could be going faster
not slower as it appears. That is why car hubcaps seemed to turn
backwards in the old flicks, in spite of the vehicle obviously going
forward. What you are witnessing is the sync of the prop with the
shutter and any variation in either can cause the illusion of
movement.

D