At Artaxerxes death, three of his sons claiming the throne*. So the Jews took advantage of the turmoil and recommenced work on the Temple. The provincial governor tried to stop them, but they just ignored him. Then the governor wrote to Darius II, when it was clear he would be the victor, to ask him to stop the Jews from continuing this dangerous construction.
* There’s no mention of the ascension in the chapter, but since the king is now Darius, it fits what happened after Artaxerxes’ death, and allowing the chapters to progress chronologically makes for a better story. However, Wikipedia does say that it’s all Artaxerxes I and Darius the Great, and the Jewish Encyclopedia agrees. But we’re here for the story, not the facts, right?!