They released the information on this durong the last clan war and it is in the news feed .. If your clan is in diamond and you do not earn at least 1 cp they will opt you out . they did this due to so many inactive clans just sitting in diamond division where people want more competition.
If your clan got demoted to a lower division, there's a chance that you may not have played previous diamond division clan wars. Mostly, your clan's previous performance from the last clan wars that you joined in affects the determining factor for placements in divisions.