Toboso Yana, the author of Kuroshitsuji manga, posted an entry about Kuroshitsuji II in her web diary. She explained the background of the decision to announce the new master and the butler last January.
Spoiler warning! Watch the episode 1 of Kuroshitsuji II before reading the text below.
When the second season was announced on January 31st last winter, she received tremendous numbers of messages from the fans. The majority of the messages were criticisms against the absence of Ciel and Sebastian from the second season. "How dare you desert the fans, who have been waiting for the two?" "If you don't have affections for your characters, you don't deserve to be the author!" She and the producers had expected the bitter reactions because they intensionally hid Ciel and Sebastian from the announcement. In the first season, director Shinohara chose to bring the revenge of Ciel to an end and Toboso had approved it. The overwhelming enthusiasm of the fans had given the green light to the second season after that. Toboso asked the producers not to reset the finale of the first season. To meet two conflicting conditions: "We must not revive Ciel easily" versus "We want to show the two once again", she had been vigorously working on the scenario for half a year with other staff. In conclusion, they had created a new rival pair, Alois and Claude. With an intension of showing them "big" and dominant, Alois and Claude were placed in the main of the key illustrations and the trailer and Sakurai Takahiro and Mizuki Nana were assigned to them. Toboso said that the aim of the tricky announcement will be understood if you watch the series to the end.