If the URL is structured YOURDOMAIN + something + YOURDOMAIN, that is usually an indication of a broken internal URL reference. The fact the pages don’t exist and transmit a 404 code rules out the possibility of a black hat spammer trying to use your site for links (unless it is really incompetent).
http://www.highrankings.com/forum/index.php?showtopic=41678
SERP Monopolies are likely to become very common within the next 3-5 years.
