Yeah, it does look kind of like that, so I suppose the photographer's movement must have been a factor as well -- he and the subject (and the guy on the right) were in some kind of sync, so their movements cancelled each other's out.
Originally Posted by Will
This suggests a few potentially interesting experiments:
-- Experiment 1: Put the camera on a tripod. Have your subject go stand in the middle of a crowd of people. Tell the subject to hold as still as possible, while the other people mill around naturally. Shoot at a wide range of shutter speeds. See what happens.
[Hypothesis: As you go through the shutter speeds, your pictures should go through a range in which everybody is sharp...then only your subject is sharp...and then nobody is sharp. But you might also get some surprises, like the guy on the right in the original picture.]
-- Experiment 2: Same setup as above, except hand-hold the camera. Tell the subject to move in a particular way and try to mirror his/her movements. (As suggested elsewhere, don't forget to try twisting movements as well as linear movements.) See what happens.
-- Experiment 3: Do the same as #2, but don't insert or direct a subject. Just pick out someone in the crowd and try to mirror his/her movements.
It could be a complete waste of time, but I'm thinking maybe some cool pictures might result. I'd try it myself, if I had any friends who would put up with this sort of thing!
[It's fun to find pictures of a crowd in which one person is different from the rest; here's one of mine:]