My guess: Since all new posts are appearing on page 1, I think that the system has "forgotten" about pages 2 and 3. They're only accessible at the moment because the system is set to write out static HTML pages, rather than call the database every time somebody accesses the page.

Therefore, my guess is that when page 1 gets a bit longer, the system will overwrite the old page 2 (which for some reason it doesn't know exists), and eventually the old page 3 if the thread gets that long. That's why I copied the items from the old pages 2+3 into page 1. Odd behavior though.