Just like we tend to categorize easily other people on basis of their physical features, dress or language, we also do that on basis of their behaviour. Taking pictures of people taking pictures is instinctive and has its appeal .
It is normally easier to catch people unaware, when they are concentrated on doing something which commands all their attention. Nowadays, we are almost back full circle to the times, when field cameras with their big ground glass needed to be set up on a tripod, and black cloth over the head of a photographer was isolating him from the outside light, so that he could focus and frame correctly. I am actually surprised, that nobody sells this type of cloth as an accessory for the Ipads.
All the small sensor cameras have also made the “selfies” much easier and more popular, as the almost infinite depth of field helps to place everything in focus, including the shooter, and he can often control the framing by looking at the image as it forms on screen. Thus, the environment probably benefits, as we see less of these incisions : “Joe was here”, made with a pocket knife on monuments, benches or trees. On the other hand, a new way of thinking is emerging, in which making a “selfie” in close proximity of something/someone of value or interest, should somehow elevate the status of the shooter through an implied mechanism of osmosis.