This is highlighting questions with tags that are in your favorite tags.  If you haven't manually added any favorite tags, the system will detect when you have a certain level of activity in a tag and infer one for you.  If you manually add a favorite tag, the system will not infer a tag for you.

You can customize your favorite (and ignored tags) in [your profile](http://pm.stackexchange.com/users/927/bartosz-rakowski?tab=prefs).  You can also customize these settings in the right sidebar of the homepage and the /questions listing.

So, if you would prefer not to have any questions highlighted, you can simply add a nonsense "tag" to your favorites.  Perhaps [tag:unicorn]? (: