I've looked into this, and the most likely culprit is an old buggy version of
vBSEO, which is an analytics plug-in for vBulletin which this site runs on.
AK needs to update
vBSEO to the latest version (and possibly the entire vBulletin install) in order to correct this.
The problem is only on clicking links from google searches that I'm aware of since they use a linkback URL syntax. It will happen once or twice or a few times in a row and then it will eventually cure itself.