I have no idea. All discussion forums should default to "Last Post Time" and the Classifieds forums default to "Thread Start Time." There is no way that I know of for you to permanently change the views, you can only change them temporarily while you're on the page and they will revert back to the default after you leave. Are you sure it's happening in every forum? There's a chance I have one or two forums setup incorrectly but nothing that would affect the whole site. What happens if you view the forums while logged out?