My thoughts exactly: by bringing two of the five fingers together you achieve a more interesting effect of overall asymmetry among the fingers, and yet because the two joined fingers are each flanked by a "detached" finger, it creates another layer of symmetry within the non-thumb fingers.
The overall effect is quite pleasing to the eye, which may account for it having caught on to the point where it became a trend. I see this as the Occam's Razor of explanations.