Traditionally if a menu can't fit in the available desktop space in LibreOffice it gets rearranged to some other location where it does fit. So first attempt is underneath the menu entry, then above, then left, then right. Which can give this type of placement.
Which is rather undesirable. For Libreoffice 4.2 for menus from menubars we will just attempt placement under or above and on failure to fit, then pick which has the most space and place there anyway and instead scroll. Like so:
Which means that you can always just move left and right after activating a menu to navigate to the next one without a poorly placed menu getting in the way.