It seems like you have an HTML cache for not-logged-in users, like Varnish, or something like that. For each users IP, it looks like it will just keep showing the first wiki page they went to over and over, no matter if they are trying to look at a new page. It may not be configured correctly, or if it's a vBulletin mod, it may need a custom integration. If you can't tweak it properly, you can trying disabling it on wiki pages.