They reappear in landscape.
I think it's something to do with the browser width and the visible links in the top menu. Interesting although I don't have this issue on my site currently (I have more menu links), I did a little while ago and I think I mentioned it in a conversation with Brogan. At the time I resolved it by renaming one of my links (from Chat to Chat Room).
*EDIT* I also think the tab that's focused is related, it's only on the Members tab here that this happens. When it occurred on my forum it was only on the Home or Forum tab (I can't renember which).