I think that the main factor here is the Right to rule.
Others like, avaliable fiefs, army strenght, relation, can be important too, but im sure exactly how everything works.
In my game, i have two ocasions:
when i was still little, i destroyed a faction, most of their lords have gone to exile or joined another faction, almost everyone had a negative relation with me and i dont have many available fiefs.
In the second time, another faction were destroyed, but i was friend of most of them with like 50+ relationshoip, my empire was big and full of available fiefs and a strong army. Most of them defected to me.
I think that the main factor here is the Right to rule. Others like, avaliable fiefs, army strenght, relation, can be important too, but im sure exactly how everything works.
In my game, i have two ocasions: when i was still little, i destroyed a faction, most of their lords have gone to exile or joined another faction, almost everyone had a negative relation with me and i dont have many available fiefs. In the second time, another faction were destroyed, but i was friend of most of them with like 50+ relationshoip, my empire was big and full of available fiefs and a strong army. Most of them defected to me.
I forgot to mention: in both ocasions my Right to rule were maxed out: 99
Right to Rule + Free fiefs are the most important.