There are more links on pages with the words "stack overflow" near the link, or with the anchor text itself "stack overflow", pointing to stackoverflow.com than to any other page on the web. Lacking additional context, the web teaches Google's algorithm that when someone is talking about "stack overflow", they are referring to stackoverflow.com more often than anything else.

If you made a website called "Nikon Cameras" today but the site had nothing about Nikon Cameras on it, it would not rank well for the search "Nikon Cameras". Other people writing about Nikon Cameras would not link to your site more often than something actually about Nikon Cameras.