I've come across a weird phenomenon of the AI. Apparently sometimes a pretty stable country will decide to just split in half forming two new tags. In my case that was a large saxony splitting into Franconia and Friesland. Also Denmark is doing it all the time.

Form, conquer most of the danish peninsula, then decide to turn into hamburg. Then turn back into denmark. Then split into Schleswig and Denmark. I'm pretty sure that's not caused by rebels.

Is this some mechanic that is to simulate the frankish inheritance customs where the land was split between the sons equally? Or are the AI in that region just particularly prone to liberate nations and reforming whatever tag just happens to be available.