I'm not sure exactly, but this might possibly be wind effects on the bullet. Arma 3 does actually model wind, and its ballistic effects.
I have not noticed if trees or other vegetation are affected by wind in the game, but if you blow up a vehicle, and watch the smoke from the explosion,
then you can see what I'm talking about, as well as get an idea of wind speed and direction.
I also don't know if wind has any effect on flags, as I have really not tested this, but IRL, snipers use vegetation motion, dust, smoke, flags, or pretty much anything else
in their environment light enough to be affected by wind to judge wind speed and direction when they don't have an anemometer handy.
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As an update to a reply I posted to this same question (has not been posted yet, because it's still waiting on moderator approval)
Wind DOES affect vegetation and flags in game. Although, at higher wind speeds, the effect is not as pronounced as it should be, not everyone will
be playing missions with wind speeds over 80 anyway (don't know if this is MPH or km/h, but I'm assuming km/h as everything else in game is metric).
Wind does NOT seem to have an affect on the various bits of debris and insects fluttering around the map. Though, this could just be me.