So I need to mouse over my threatened planets to see what the state of their influence is each turn?

Could it be that the one benevolent feature that flips all starbases and planets in one's sphere of influence was the cause?

Does anybody know if the AI is also subject to planetary and other events and so gains ideology points that it can spend for benefits like the player can?

I also notice having gone back that there was a little red icon above my planet before it flipped but since I play zoomed all the way out I didn't notice it.

You can tell when one of your planets will flip.... the first chance it gets! If its an AI planet,, well then it will be an exceedingly long slow drawn out process.