I've gotten these from multiple web sites, including Drudge and National Journal.
It is my opinion that this behavior is a side effect of the latest Java update. Until this latest update, Apple has maintained its own version of Java. The latest update is Oracle Java.
Apple's note on the Java for Mac OS X 10.6 update 11 includes this:
On systems that have not already installed Java for Mac OS X 10.6 update 9 or later, this update will configure web browsers to not automatically run Java applets. Java applets may be re-enabled by clicking the region labeled "Inactive plug-in" on a web page. If no applets have been run for an extended period of time, the Java web plug-in will deactivate.
Looking at the innards of one file downloaded, it appears to be a JavaScript beacon from advertising.com for rubiconproject.com. These files are used to track web browsing activity. I suspect that this Java changed the way the beacon is served and Safari.