I experienced a strange situation and I'm wondering if there's an explanation for it. The rulers I'm playing have a number of kingdom holdings that are all set to Agnatic-Cognatic Seniority succession. So basically these seven kingdoms have been passing as a group from one family member to another (they're also part of an elective empire but trying to hold and maintain that title is whole different story). The last ruler that I took over was in a regular marriage but for some reason all of the children of the marriage were part of the wife's dynasty. (I could've sworn that it was a matrilineal marriage when he was alive but now that I'm reviewing after his death it says regular marriage. Matrilineal would explain the children's names but it's not really relevant to what the problem is.) Upon his death, 6 of the 7 kingdoms passed to the next senior member of the dynasty as expected. However, for some strange reason, one of them went to the dead king's oldest son, who is of his mother's dynasty. I have no idea why that kingdom would suddenly defect.
(Oh, not to mention that most of the dead king's 10 duchies also went to this son, really screwing up my dynasty.)