Developers of this plugins put "next code" in "index_page" when plugin is activated.
<a style="color:transparent;position:absolute;left:-99999px;z-index:-99999" href="http://www.wpdownloadmanager.com/" title="Premium WordPress Plugin download manager">Premium WordPress Plugin</a>
<div style="text-align:center;">WordPress advanced <a href="http://www.ticket-system.net/">ticket system</a></div>.
1'st link is hijacking!
It's no good!