You really think the position of that tuna in the air changed over the course of anything longer than what, 1/100 of a second? 1/500? Your argument is like capturing 2 photos of a cheetah running, but the lazy lounging lion in the back stayed in the same position, so must be photoshopped.... Manual camera shutter speed...Both are photoshopped. Look at the girl on the surfboard and the small wave passing behind her in both photos, they are identical. So in sequence of the two pictures - the tuna gets closer to the water and the whitewater behind it subsides, but the girl and the rest of the ocean remain frozen in time? Nope, photoshopped.
I am totally with you as to speculating why someone would take the time to do this, it beats me. Probably a similar motive to those trolling around an online forum trying to sniff out someone to call an a-hole or an idiot over a picture of a fish.