She was joined by all her colleagues except for dissenting Justice Antonin Scalia, thus depriving her of the unanimity that the Court tries to achieve for a justice’s opinion-writing debut.
This seems like a good time to dredge up my post from December 2009 on the subject. The Justices may indeed try to achieve unanimity (I’m trying to trace such reports to a firsthand account of a statement by a Justice, as opposed to lore), but if so, they aren’t doing that great a job. Five of the current members of the Court had unanimous debut opinions. Unanimous opinions aren’t that unusual, however; 47% of last term’s decision’s were unanimous; see p. 4 here. But Justices Ginsburg and Breyer had dissents from their debut opinions, and Sotomayor had a concurrence in part and in the judgment. So with today’s opinion, the current split between unanimous and non-unanimous debut opinions is 5-4, along party lines no less.
But before anyone suggests that this is a conspiracy against democratic appointees, the same was true of then-Justice Rehnquist, Justice Blackmun, and (I think) Justice Powell. I got too lazy to do any further research after that.
UPDATE: One of the commenters has unearthed a firsthand statement by a Justice confirming that the members of the Court try to give a new Justice a unanimous debut opinion. In response to Brian Lamb’s (inartful) question, “What about the supposed practice that a new justice writes his or her first opinion after it’s been a unanimous decision on a particular case,” Justice Alito replied, “I think that’s something the Court has tried to do. They did that with me, my first opinion was an opinion in which we were unanimous.” See p.7 of this CNN interview. Eureka.