Dear Arne,
A large number of topics for your plugin get "put" into the Installation forum. It probably is a result of the links you have in your readme file that say <a href="http://wordpress.org/tags/google-sitemap-generator">Support Forum</a> and http://wordpress.org/tags/sitemap When a user clicks on that it shows the threads for your plugin, but the Add New Topic form defaults to the Installation forum.
Any chance you would consider using this link, http://wordpress.org/tags/google-sitemap-generator?forum_id=10, instead, in your readme.txt file? Or this link http://wordpress.org/tags/google-sitemap-generator?forum_id=10#postform
Also note the tag for this plugin is "Google-Sitemap-Generator " so you might also change that.
Thank you for considering this request.
http://wordpress.org/extend/plugins/google-sitemap-generator/