Rolling shutter is perhaps the biggest drawback of most CMOS sensors. Because the entire image isn't capture at the same instant, but is, instead, "scanned" from top to bottom, anything with significant movement during this period will be distorted in the final image. And it is rotary movements that produce the strangest effects:
Kudos to Anton River for devising such a simple way to explain this!
(RedSharks more detailed explanation of Rolling Shutter here)