The reason (I think) is because it loads the page when it sees the first next link on the page. The first link it sees is at the very top of the thread, so it figures you are at the bottom of the page. Since both the top and bottom next links are identical, I'm having trouble telling xpath to get the second one with the // search type.
edit: Here's a fix:
(//a[text() = 'Next' and contains(@title, 'Next Page') and contains(@href, 'view')])[2]
This selects the second next link only. The other thing I had to do was in site manager in the advanced tab for your rule, select 0 for the minimum preloaded page count. I think it defaults to one preloaded page, which causes an auto load.