They are elective in ACoK - which makes sense. But they revert back to their normal succession after first Kingsmoot IIRC. Which also makes sense, as Kingsmoot isn't your everyday's happening - and Seastone Chair is mostly inherited by primogeniture / 'open' (arabic) succession.
So, they are elective... Once, and then primogeniture? It isn't everyday, but i thought everytime their leader died they had a kingsmoot?