From what I know, there's only one way for an adult to change (a decision under the intrigue tab where a character changes culture to that of his liege) and that decision includes a line for AI where it is given a factor of 0, which means it should never be used. This probably means that there's some AI specific means of culture shift. I'll look through the event files and see if i can find it for you.
An event occurred to my new Basque (yay!) king. Something along the lines of "I feel close to my peasants" and the options were A) upset all my Basque relatives and convert to Irish, or B) do nothing. This has only happened once, and I have had a series of Basque kings in Ireland for a while now, so perhaps this event fires more often for the AI, or more often if your liege is of another culture. Or maybe I was just very unlucky.