Well, when there is a new post in a thread, the thread moves to the top.
That's just how the forum works. I know it's not what you would expect for a news system though. But the only viable way to circumvent this would be to block replies from the Announcements forum.
Hmm, or maybe we could just lock the topics when they get old. Yes, I think that what I'm going to do. Thanks for the suggestion!
(Please note that this only applies to the Announcements forum. For all other forums it's expected and good that topics with a new reply move to the top.)