In my first game with the Charlemagne DLC (starting 769) King Charles Karling the Great created a pretty big realm. But then suddenly, in 791, many of his vassals, including the duchies of Brunswick, Saxony, Thüringia, Franconia and Burgundy, became independent:
I didn't see any independence revolt and I'm also not aware of any other mechanic that could have caused this.
So, what happened? Is this something specific to Charlemagne, the character? Is there something I could do to investigate this further?