Heir flip-flopping

You likely have some sort of elective law. Opinions change over time, so the person who would win the election today is not necessarily the person who would win the election tomorrow.

Mr. Actinguy is right, you have an elective monarchy and in an elective monarchy a certain number of your vassals are 'electors' (I think each duke?) and they can cast a vote for the successor to the kingdom. This doesn't necessarily have to even be someone related to you. You also have the opportunity to 'nominate' a successor in the laws tab. This will put your own vote(s) behind that candidate, and also makes the other electors more likely to vote for him as well if you have good relations with your vassals. If you don't nominate a successor then its left pretty much up to your vassals and, like Actinguy said, their preferences change over time.

Actinguy said:

You likely have some sort of elective law. Opinions change over time, so the person who would win the election today is not necessarily the person who would win the election tomorrow.

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This. Sweden was not a Kingdom of "Oldest son inherits" at the time. For example in the Christian-Ásatrú Civil War that historically started shortly after the start of the game the incumbent King was deposed and the people elected an Ásatrú ruler to stand up for their rights, since Christian Zealotry (persecuting people, making religious duties illegal etc.) was beginning to take hold in the majority-Ásatrú east and north of the country. The Christian former-King didn't give up, though, and rather than accept the decision he became a pretender and raised the Christian South-West into war on his behalf. He eventually won through assassinating his enemy and his family via a Quickfire in the night.

So not only was the inheritance power weak, you could essentially have your King-title revoked and become a pretender if enough Lords and citizenry thought it needed to be done.