A large number of topics for your plugin get "put" into the Installation forum. It probably is a result of the link you have in your readme file that says
http://wordpress.org/tags/yet-another-related-posts-plugin 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/yet-another-related-posts-plugin?forum_id=10#postform, instead, in your readme.txt file?