So there I was, expanding merrily as can be as Advent Rebels, just grabbed a nice little moon in a pipe towards my nearest enemy.
At this point my Loyalist teammate mentions he's just gotten his automated culture spreading from planets he owned, and that's when I noticed this happening:
His culture was spreading from my moon, which was on the complete opposite side of the map from his territory, and was pushing all of my culture back, so I ended up having to choke it out with a starbase with evangelism nodes in an adjacent well.
I have absolutely no clue what caused this, I just came across the screenshot and remembered to make a thread about it.