From the article, under "Unfollowing": "You have to unfollow the people you follow for two reasons. The first is that you cannot be following over 7,500 people

From the article, under "Unfollowing": "You have to unfollow the people you follow for two reasons. The first is that you cannot be following over 7,500 people at any time. The second is because — although artificial — you want to have your follower/following ratio as high as possible as it is a sign of a more desirable account."