You can't really say this is the behaviour of menus "in Linux" because there are many desktop environments you can use in Linux, which may have many different behaviours regarding menus. Vanilla Ubuntu uses GNOME, where a context menu appears when you press the mouse button. If you then release it, it remains, and you can make a selection by another click. If you hold it down, you can move the cursor over an item and then release to make a selection. A quick test shows that there's a delay for activation of this "release to select" behaviour, so this perceived "randomness" only happens if the click is _very_ slow and ends up being interpreted as a hold instead of a click.