The surprising thing about this photo, really, is not that they found a guy who was yawning, but more that they ever manage to take a shot without someone messing it up. Consider every time you try and take a family photo, with about ix people in it, and how there's someone always messing it up. Then multiply that by 100 for a full parade.
Of course they practice this kind of thing in the army, but you can't beat human nature every time, can you? If you look at it more closely, in fact, it looks like he's deliberately looking down the barrel of the camera and yawning on purpose. Whether intentional or not, he certainly managed to stand out that day. It was even better a second later, when everyone else caught his yawn.