A forum post by a Blizzard community manager explains that recent shifts atop the leaderboards for Diablo III are the result of a round of bans for those caught cheating in the action/RPG sequel. The post explains they will continue to take such actions going forward, though they may not always be publicized. Here's word:

Some of the shifts you may be seeing on leaderboards are a direct result of a recent ban wave issued for cheating.

Players that were found to be in violation were removed from the leaderboards and other rankings have been adjusted appropriately as a result. This means your ranking may have increased if someone ranked above you was removed from the leaderboard. As we move forward to future seasons and eras, we'll be continuing to monitor for such behavior and take appropriate action, though we may not always message when such ban waves occur.