So, it's a bit odd that the Crown in Twig even has a line of succession.
It does however make total sense that they would utilize it as a chance to pick out the weak elements, those who are not smart or strong or weird enough to survive or triumph.
I thiiiink that King Adam may have instigated this, and that he is still the original King, immortal and slowly having alterations done to his body over decades to see if they can remove all vulnerability. He probably doesn't care if he is eventually killed, instead seeing it as a possibility to remove a weakness and have something even better take his place.