Given your clue, I started to wonder if it was something to do with how sensor's might record movement differently than film.
Then I noticed that, on careful examination of the chopper images, it is in fact only one of the blades which appears to be markedly curved. Hence I deduce that it's to do with motion blur, more particularly the direction + viewpoint of each blade, i.e. the blade which seems to be most affected (curved) is the one which is
most parallel to our viewpoint + it's closest to us + it's direction of movement is
directly across our vision, so that the blur has changed the apparent shape of the blade, which is accentuated by the change of colour along the blade, i.e. the band of yellow against the band of dark colour, which appears to give an angular effect in the shape of the blade.
My version doesn't seem to relate only to DSLR's, and I could be way off the mark, but this is my guess anyway
