Much has been made of the fact that Justice Souter did not follow the practice (of Justice O’Connor and Marshall) to agree to remain on the Court until the appointment of his successor. Instead, he stated his intention to retire “[w]hen the Supreme Court rises for the summer recess this year..”. Justice Souter made the announcement on May 1, though he may have tipped off the White House earlier.
Citizens United was argued on March 24. By May 1, it could have been that the Justices were struggling with an opinion, and the outcome was uncertain. Or it could have been that the Justices knew by then the case was being reset for oral argument, perhaps because Justice Alito wanted more than a couple of pages of briefing from the government on overturning a law that (in various incarnations and with different coverages) has been in place for 100 years. My guess is that by May 1 the Justices knew an opinion was not coming and the case would have to be reset. On this theory, they delayed announcing it until the end of the term because nothing would be gained by announcing it earlier—there was enough controversy at the end of the term in any case.
The pendency of rehearing in Citizens United would provide an additional reason for Justice Souter to get out of town in June. In the event his successor would not be named by early September, he’d participate in an argument but leave the Court before an opinion would be issued (likely after the appointment of a successor by the first Monday in October, which is what Republicans eventually began calling for). His participation in argument but not decision might have been awkward, not to mention being a source of major heartburn.