Well, if you did have 100 relation with all of them prior to them defecting you, it does sound strange.
Only thing I can think of is personalities (if the lords defecting you were dishonourable) and the Aserai were paying them good money... at the end of the day, it's the same thing you can do to them back and forth... it's all about the money. With 100 relation I would assume an honorable lord wouldn't defect you, but I suppose dishonourable ones will not care for relation value if offered enough money/fiefs?
Which to be honest leads to another point: It shouldn't be too easy to be swimming in gold, and it shouldn't be too easy to boost relations, it should take effort with quests and careful management of political choices and assisting who you want to befriend in battle or by any other means (the more, the better).
And it should obviously apply to AI as well. As a whole, it should be much harder to have a lord switch side, and not just a matter of having enough cash and winning a % chance with the charm skill, which if I am honest doesn't really add anything at all to the game.
It would be much better, as it would at least require a bit of planning, to force a player having a minimum relation with a lord you want to convince to join you prior to attempting to convince them, and then having to choose the right options based on their personality, not going for the one with the highest % being told to you by the game.