Perhaps because they see a funny shaped fish with a huge freakin' eye! Notice in nature how some fish have black (or dark-color) eye-spots to make them look bigger. This might have something to do with it.
I got the picture of my fish in my gallery by using a decoy. I cut out a cichlid-shaped piece of paper and stuck it to the glass. Interestingly enough, when I first made the "fish", I colored it bright yellow with a marker, and the fish left it alone. I flipped it over, so that only the color that bled through the paper showed and it was more pale, and the fish displayed aggression and tried to eat the paper, and I got my picture. Of course, this will only work with aggressive fish.