By default, forums are configured to distribute email to interested members for any online post made to any of the forums. By default, new members are configured to be "interested".
To change your settings so that you no longer receive emails from forum posts outside your own, see the FAQ entry "I've posted a thread to a forum, and now I am getting notification emails for any posts to this forum and others. How can I stop or control this?" under the "Navigating the Website" category.