There is a variety of possible explanations, but the least likely is the one claimed by the conspiracy theorists, air.
For one, the flag, a very light, as can be seen by its translucency, nylon one, does not billow but maintains its folds and swings back and forth in more a pendulum fashion than any flag in air would. Moreover, the video also shows the astronauts kicking up dust as they pass in view, which settles quickly without aerosolizing, which again is vacuum behaviour.
Finally, the flag takes a considerable amount of time to stop moving, again behaviour to be expected of a vacuum environment. Drag on something with such a low mass, high surface area as a nylon flag in air would bring it to a stop quite quickly I believe.
So whether it is electrical charges, vibrations up through the pole, or the astronaut simply brushing it at some point, air in particular can be pretty safely ruled out in my opinion.