By default, topic tooltips used to be permitted and were a new feature of the 7.x software. I was requested to remove it from the template by many of the users for various reasons (annoying, browser compatibility, etc.).

It's now a board-wide configuration where the admin can determine the number of characters to preview, however it's presently disabled for the reasons mentioned above. I have a feeling someone may have been experimenting at the time you saw the pop-ups.