Why was Justice John Paul Stevens able to select the author of the Lawrence v. Texas majority opinion?

In the case of Lawrence v. Texas, Justice John Paul Stevens was able to select the author of the majority opinion because he was the most senior justice in the majority.1

Under Supreme Court tradition, the chief justice holds the authority to assign the writing of the majority opinion if they are part of the winning side. However, if the chief justice is in the minority—as was the case in Lawrence, where Chief Justice William Rehnquist dissented—that authority passes to the longest-serving member of the majority. As the senior justice among those who voted to strike down the Texas law, Stevens exercised this power and designated Justice Anthony Kennedy to write the opinion.231