Over the past couple of weeks, we've noticed that the search results from Google for some of our drupal-powered sites are having their page titles hijacked somehow. An example would be:

free streaming porn - [Actual page title]

There are other variations of the porn prefix, that's one of the more tame ones. I looked in the databases for each of these sites and the titles haven't actually been changed or anything along those lines. When you click on the result to visit the page everything looks normal (sans porn stuff). Would anyone be able to point me in the right direction as to what the cause of this is?