Wordfence itself needs to use admin-ajax.php frequently to show progress without reloading the entire page -- so Wordfence is working as it is supposed to -- but the css file is definitely odd, so it caught my interest.
I thought in the initial log that you posted, that your browser was loading both files since the requests were from the same IP, but I tried out the link library plugin on a test server and found that the css is being read over the network by the server itself! It also appears in the logs on any other page that uses ajax, but most of those are less often. (If you sit on the plugins or users pages long enough, you should see it in the logs.)
Anyway, I tracked it down to the file "link-library-defaults.php" in the link library plugin -- specifically, these two lines:
$stylesheetlocation = plugins_url( 'stylesheettemplate.css' , __FILE__ );
$genoptions['fullstylesheet'] = @file_get_contents($stylesheetlocation);
I don't know if there is a reason that the author of the link library plugin does it that way, but if he/she can make the plugin read the file directly from disk instead, it would stop the extra hits to the web server and should be much more efficient.