It doesn't 'fly'.  It's the vibration from putting the pole into the ground.  With no air and less gravity, there's nothing to stop the effects of the vibration on the flag, hence the 'flying' effect.

Do you seriously think if NASA faked the landing they'd make such a basic mistake in filming it in a windy area?

Dig up the video I posted and you'll realise what a load of hokum the 'faked it' theory is, or in fact just re-read what I posted earlier.  That should do the job.