Answer:
People on the cover are usually there for outstanding performances. Because they are so far from the mean, the performance in the next year is likely to be closer to the mean.
Explanation:
We are told that frequently, their performances then falter somewhat, after they are pictured on the cover of Sports Illustrated."
Also, we are told it's common for phenomenal rookies to have less stellar second seasons, which is called "sophomore slump."
Now, this could be due to pressure and the need to prove themselves.
Now, someone to be on the cover of Sports Illustrated, it doesn't neccesarily imply that such a person is the absolute best. It just implies that their performance was far better than how they normally perform. This may be due to either real skill, heavy training or a luck.
However, the most comprehensive reason is that;
People on the cover are usually make it there due to their outstanding performances. Thus, because they are so far from the mean, the performance in the following year is likely to be closer to the mean.