It's similar to the reason that photo-finish cameras in sports produce such distorted images:
Usain Bolt wasn't really wearing a clown shoe at the London Olympics. His foot appears distorted because the camera is capturing a tiny slice of the image (the finish line) over time.
If you look at a freeze-frame from a normal camera, you can see that Bolt's foot happened to land right on the finish line, so it was stationary for approximately one tenth of a second and so got smeared out into a long line.
And why does the background of the photo finish picture look white, with black lane markings, rather than red, with white lane markings? Because the entire width of the image shows the finish line itself, with the x-axis being time, not distance. This photo shows the little black markers on the finish line that create the lane divisions in the photo finish. (The red-and-white "stick" on the far side of the track is actually a slit that reveals the sponsor logos on a rotating wheel, synchronised to the speed of the camera, creating the Omega/London 2012 images you see above.)