How did my dynasty change culture?

[Wombats O' War]

Playing a game as France, I suddenly noticed my new King had "Germanic" culture instead of Frankish. How does this happen so I can avoid it in the future? That foreigner penalty to all my vassals is awful.

ack, way wrong forum. PMing mod

 

[unmerged(174159)]

Just on topic briefly - cos your heir was raised abroad perhaps? Or he took after his mom?