if you look closely, you'll notice at some times a dark area inbetween the blades, just slightly darker than the rest of the sky.
The blades are rotating, the shutter is giving it the same effect a strobe light gives your ceiling fan....
if you don't know what I'm talking about, go buy a strobe light and mess with the speed setting with the lights off in your bedroom until a fan (ceiling fan or other) seems to come to a stop despite it being on. The air is still there as you can feel it, you just are getting the illusion that the fan is stopped.
Same thing here. It's a coincidence that it worked out this way... I doubt it was intentional.
I remember when I was a film student (before I got an interest in business and economics) when we had problems with takes because of computer screens, televisions, fans, etc.