I think it's the first time I've seen that popup. I assume someone won a game that pushed me down the map rankings on a map where I had champion points. I clicked on it, but it gave me no further information. Is there an (easy) way to tell what happened?

https://www.wargear.net/players/info/Ozyman/Ranking%20History

Those look like they are my wins & loses, but I think I actually got demoted not because of a game I played, but because of a game I wasn't in. I think someone won a game (that I wasn't in) on a board that I was getting Champion Points for based on my ranking on the board. Their win pushed them ahead of me on the board ranking, and so I lost the Champion Points.

Every board has a Ranking History tab that shows a graph of the up and downward movement of the top players, but it doesn't seem to be working now.

You likely got demoted from a game you were in - Your last completed ons on Pirates of the Caribbean. The loss sent you from 6th to 7th in the board rankings, and down from 6 to 4 CPs there.

Could of course be somewhere else as well.

You only need one more CP somewhere to get back to Major General.

So on your home page on the top of your Game Events ticker, all the way to the right is the championship point changes, should show recent activity there after a click I would think?