It appears that, if you have no weapons other than a launcher and a binocular, switching fire mode for your launcher will cause you to swap to binocular. Or if you have binocular equipped and for some reason pressed the "Next Weapon" key, it will cause you to switch to launcher weapon. It's first noticed on launchers with multiple attack modes but apparently it affects launcher with only 1 mode as well. Not sure if it is intended behavior there.