This is not to toot my own horn here (I would of expected graduations as well due to the sky and the ground), but I think I would of said It would of tended to orange, on the grounds that (and this is just my suspicion) people find a more orange hue a more pleasing whitebalance. What is online is what has been shared, and what is been shared will invariably be pleasing to the people that shared it.

Obviously, this is just another explanation.

Actually I would of liked to of seen some of those aggregate photos. Maybe there WERE graduations!

edit: wish granted!: http://jbum.com/papers/EmergentOrange_paper.pdf

some of his sample sets did produce graduations, in particular vignettes and lighter at the top darker at the bottom themes.

in defence of my statement is figure 10

  Figure 10. Averages of four different pools of digital 
  abstract art found on Flickr. Pools used: Processing, 
  CGArt, Computer Art Creations, Generative and 
  Evolutioanry Art

for some reason he produces a surprisingly orange result, and still believes that the overall orange hue may be due to:

  4) It’s caused by chemistry & physics. The amalgam photos 
  are functioning as a kind of mass spectrometer, 
  reflecting the average chemical composition of the
  subjects being photographed.
  
  [...]

  I find #4 the most convincing. It is not necessarily 
  mutually exclusive with some of the others (e.g. the
  chemistry of the earth/sun affects human choices and 
  camera design).

Meh.