It has to do with digital photography (vs. film where everything is captured simultaneously) and the way the sensor scans the picture as it is taken.
In this example, let's say that it scans from top to bottom and the props are rotating counter clockwise. As it scans the top of the prop it captures it at one point, but as it scans the next line down the prop has moved causing a curvature. As it works it's way down the blades that are perpendicular to the scan show the most curvature, and the blades most parallel to the scan show the least curvature. As the scan hits the middle of the blade the effect is reversed because the blade is now rotating the opposite direction of the scan.
There is a name for the effect but I can't remember off hand...
(edit... forgot picture)
