Actually it is in fact the way the forum formats URLs when entered in posts.
A link like this would redirect the current page to Google:
Whereas this would open a new tab to Google:
<a href="http://www.google.com" target="_blank"
You can try for yourself here: http://www.quackit.com/html/codes/html_link_code.cfm
Generally, websites use the first type of link when pointing to pages on its own site to prevent clutter, and use the second type of link when linking to content on a different site. The second type allows webmasters to keep visitors on their own site (by retaining an open tab), while still allowing them to go browse other sites. And when the user is done with the other site and closes the tab, they're greeted with the tab they were browsing earlier.
The middle-click new tab and right-click new tab features were created specifically by browser makers to address this organizational issue, though most websites nowadays have struck a good balance of when to use the first or second link type.
Yeah I know, sounds maybe a bit picky for such a small thing, but as a former webmaster myself, I just thought it strange to find a site in 2012 that uses purely the first link type, and especially on a forum where links and such pop up in the middle of discussions all the time.
This link covers the details on how it's implemented on a phpBB type board:http://www.phpbb.com/kb/article/links-o ... w-windows/