I think trending in that context means 'traffic for these terms is much higher than usual' rather than 'these are the most popular search terms'. At a guess, t

I think trending in that context means 'traffic for these terms is much higher than usual' rather than 'these are the most popular search terms'. At a guess, the most searched for terms will be dominated by perennial brand keywords like 'Facebook' and 'CNN', so rarely change.