[KubricksOdyssey (OP)]

[CliffordK]

It appears as if the flag has a rigid top bar, and is designed to deploy with a little slack in the bottom to give the appearance of ripples.

It seems to be very sensitive to motion, with little dampening from the atmosphere.  Watching the full clip from the beginning, the flag waves quite a bit when first set in place, then settles down for most of the clip. 

At the end where you have it marked, the flag is almost completely motionless until an astronaut moves in front of the flag, at which point it starts swinging slightly, but even by the end of the clip, the motion appears to be stopping. 

The conclusion is that the astronaut who moved in front of it likely bumped it.  I don't see an intentional bump, but usually it is pretty rude to walk in front of a camera, and since the shot is all about the flag, why else would he have walked between the camera and the flag?

[alancalverd]

In the absence of air damping, any vibration will set a flag oscillating. It's the absence of air that gives the impression of fluttering in this case.

[evan_au]

The period of oscillation should be higher on the Moon than the same flag on Earth.

It appears to me as if the flag moves towards the camera when the astronaut moves in front of it, rather than away as one would expect from a breeze caused by an astronaut-actor moving on Earth - or if the astronaut brushed against it.

Could this be due to electrostatic attraction caused by hopping around on the dry surface of the Moon? [...but there does not appear to be electrostatic attraction when they are moving near the flag, earlier in the clip...]