Halfway into season one of the anime the storyline changed from that of the manga and new characters were introduced. Then, because of the changes made in season one, the second season was also completely different to the manga. Book of circus, season three, picks up where one veered off. Was there a reason why they didn't just stick to the plot of the manga to begin with?
At the time the first season of the anime was developed, there was not enough published manga content to fill a season.
The first episode of the Black Butler anime aired on Oct 2 2008. (IMDB) At that time, the last completed arc was the Curry arc. The first chapter of the Circus arc was not published until August 18, 2008. (Kuroshitsuji Wikia)