Reading tea leaves....
The Supreme Court divides up arguments into "sittings," each of them two weeks long. McDonald v. Chicago was argued during the sitting of Feb 22. The Court's custom is to make sure each Justice writes at least one opinion from each sitting.
The Feb. 22 sitting had 13 arguments, so four Justices will write two and the other five one.
From that sitting, two Justices (Kennedy and Sotomayor) have already written two opinions, so they're not in the running. Stevens and Scalia have each written one. There are seven cases left to be decided.