I use + and -, but switched it to the Numpad and the problem persisted. Switched to the mouse keys, problem persisted.
It's definitely a reproducible bug. I use right click to toggle look down scope; Using - and + (and others when I reassigned them) I'll zoom in and out. After hitting - to zoom out (or holding it) the weapon will be thrown as though I hit G. It does not occur every time, but it occurs when using the zoom out button.
It occurs while crouching or standing. Haven't tried while prone.