Well, I don't really have that much input as to what might have happened, but I had two similar situations happen to me.

At one time, my king had a 14 year old son, died in battle, and my son became King that year. I'm happy to say, that 14 year old son lived to be in his 70s and produced a fine stock of heirs. Seven to be exact and the worst one of them was a 4 star general already. I had fun with his sons, although one of them died in battle. He just had to be the first one to rush into the castle *shakes head*

And then another time my king died in battle with no heirs, but a 7 star general that I had married to my princess became King. I didn't mind, because I kept on going in the game :)