This is a result of the way the camera shutters operate. Usually, the cameras don't capture images in the same way as human eye i.e. they don't take image of the whole scene simultaneously but rather 'scans' horizontally or vertically. In normal cases, this rolling shutter method is not a problem, but can cause issues when the object is moving at high speeds.
Quote from user FGK:
The plenomenon is due to Rolling shutter (also known as line scan) is a method of image acquisition in which each frame is recorded not from a snapshot of a single point in time, but rather by scanning across the frame either vertically or horizontally. In other words, not all parts of the image are recorded at exactly the same time, even though the whole frame is displayed at the same time during playback. This in contrast with global shutter in which the entire frame is exposed for the same time window. This produces predictable distortions of fast-moving objects or when the sensor captures rapid flashes of light.
In other words, it is due to camera rather than the propeller blades themselves. The effect of a simulated rolling shutter on a spinning disc shown below:
"Rolling shutter effect" by Cmglee - Own work. Licensed under CC BY-SA 3.0 via Commons.