Why would Sanballat respond to Jerusalem’s rebuilding project with becoming “angry” and even be “greatly enraged”?

Word got around that the people of Jerusalem were rebuilding the city’s broken walls. From his perch in Samaria, Sanballat had felt no threat over the years from that broken-down city. Now that its walls were being reconstructed, however, Sanballat did feel threatened. While we can undoubtedly understand why that might be the case, we need to respect that Nehemiah does not elaborate. He asks his readers’ attention instead to the painful consequence of this governor’s anger.