It's based on the order of the items in Includes_Home_Horizontal.xml and then in the section that starts:
Code:
<content>
<item id="1">
<description>music</description>
It is going to focus on the third item in this list that is visible. So it sounds like you disabled Music or Videos. You can move (as an example) HD Videos above Movies and it should do the trick for you assuming you do not want to re-enable Music or Videos. Just be sure to grab the whole item:
Code:
<item id="4">
<description>hd movies</description>
<label>$LOCALIZE[31280]</label>
<icon>special://skin/backgrounds/HDmovies.jpg</icon>
<thumb>$INFO[Skin.String(CustomHDMoviesFolder)]</thumb>
<onclick>XBMC.ActivateWindow(10025,special://skin/playlists/HDMovies.xsp,return)</onclick>
<visible>!Skin.HasSetting(HomeMenuNoHDMoviesButton) + Library.HasContent(Movies)</visible>
</item>