so why would he need to manually pull back the charging handle to release a spent shell?
In this scene I don't think he does, per se:
Just looked at the scene again - it looks like it is a plot point to make a loud noise.
For some reason.
Looks like it has less to do with the weapon and more to do with the edit of the scene.
The guy is applying so much pressure on the foregrip that he has activated his light - is that intentional?
Really looks like the director told him to make a loud noise for reasons of story.
You do not get a chance to see the casing extracted so it is not the point of the shot. If you go frame by frame you see the brass glint in the light but the ordinary viewer is not going to see that. So that is not the point.
The layman would have no idea what he is doing and even less that the casing flying is not even shown.
Regarding his possible activity, if not for the story:
Not necessarily the right answer, but could give a clue:
Its very brief, but pulling back is one way to clear a jam.
It does vary greatly in the industry, but firearms used in filming are not firing live rounds - usually.
Because the weapon itself might be modified in some way (many ways, springs, buffer, slide, barrel, etc), or the rounds used are for stage use or might be practice rounds - ie. blanks, and that there are variations in the blanks themselves - some are low power, low noise for example - anything less than a full load round inside a weapon capable of more than single action are prone to suffer from feed issues, ie. the cycle does not perform fully, the case might not eject, it might partially eject, double feed, etc.
I am not wording it in the best possible way here, others will do better, but the gist of this is that:
the background person in this scene might be experiencing their weapon suffering a jam and is carrying out remedial action to correct it.
and the cause of that malfunction is likely to be the items that he is using with which to perform that action for the scene.
and this is not live fire, so do not expect the same actions to occur as in real life. (ie. use of blanks providing insufficient pressure to cycle the action)
and, as ever, there are similarities and there will be things that happen that might be unexpected.
I have also seen extras who are given weapons and do not really know how to use them and just do what they saw on tv (cue: endless racking back of slides and bolts on set) - or ignore the instructions and advice they have been given.
With regards to the real thing I have done such action to clear the weapon, and I used to own prop guns that were sometimes used in films, and they suffered from jams and inconsistent cycling terribly, and I have been on sets recently where they have low noise blanks that have jammed and failed to cycle repeatedly.
But there are better people out there who will write a much clearer and better answer than i have.
I originally deleted this because I think it is story/edit issue, nothing to do with the guy and his weapon.