I think in this case it was an accident. My son was leading troops in battle, I probably should have forbid him from being a champion but that isn't the viking way.Sometimes it's not an accident. I don't employ ones with negative traits no matter how high their skill is in CK2.
Having intergenerational plans fall apart happens a lot. I've gone from count to emperor in 1 generation but never been able to repeat it.
I'm crap at internal politics, easier to ally foreign powers and over throw your liege that way.
There is still so much to learn, It was a while before I figured out how to change succession rules for my realm which doesn't seem to affect succession rules for lower titles. I also seem to be able to adjust feudal requirements with my subjects, possibly only the ones I recently conquered only did it for one.
You can get hooks on people to blackmail them or call in a favour, that can help with elections to get your son in the seat of power, but I always end up getting hooks for people who have no bearing on the election. I rarely scheme (other than trying to murder my brother to inherit his duchy) so haven't managed to blackmail people into joining me in plots.