I frequently engage enemy vehicles while firing from hilltops or behind terrain.
As such, after firing the RPG-42 especially, I drop to the ground to dodge machine gun/grenade fire and avoid being spotted.
However, from this position, if I accidentally hit the 3 key. My character returns to a standing position.
With all due sincerity, I love this game, but little things like this drive me crazy.
What you mentioned is not a vanilla feature. You can't go prone with the launcher in your hands in the base game. So blame the mod you're using (possibly ACE)
The only thing I could reproduce was the player standing up when he's prone, and switches from No Weapon to the Launcher.
I just loaded Arma without DLC and mods and tested this in the virtual arsenal and can confirm this. However, I'm not using ACE or any mods that claim to affect RPG/character movement. I will check to see which mod is causing the behavior.
I would like to amend this simply to : Player stands up while selecting RPG while prone. I'm probably hitting the 3 key by accident.