The Winners' board is updated live, so as more people take the quiz those who performed better will be moved up in the leader board. So if you have gone down the leader board it simply means that others have outperformed you.
To climb back to your choiced possition, you need to enter more games (if you still have the privilege). You can only retain your position at the last second of the contesting period.
Why, does, my position, the winners board, keep changing