First of all, don't let your capital unprotected. Build the cheap fort and leave some army to be raised or purchased there. In emergency, give your capital to someone to move it, before enemy arrives to it.
They don't generate random seed all the time. If your capital meets the seed chance, it is likely to get affected of the higher chance. When one of your family member is captured, it is almost always 100% that all of them will be captured - all your spouses, concubines, and children.
You see, I try to sack major vassals of my enemy kingdom, especially those who has the kingdom's de jure counties. I first send my raiders to capture my enemy's top duke's wife.
When I get his wife, I make her my concubine, land her, and execute her so his child will become my vassal. Then I give the child an empty kingdom title or two to raise his opinion to 100, and imprison and execute him too.
Someday I will try playing a Norse game again, and try to conquer the whole world almost without wars.
Now the former enemy duke is my vassal king, inheriting a higher tier kingdom title from his son. If that gave me 51% of my enemy's de jure counties, I usurp the kingdom title and make a vassalization offer to the my enemy king.
If it didn't work and I have to make a war with the king, I still try to sack the burning castle again. If things go right, I could surrender and usurp his kingdom title. If not, he is still weakened and there are less counties to siege.
Of course I sack the enemy king's capital as well, and try to make his child son my vassal king. I may get to assassinate the king or capture him in the battle, at which time I can just kill the king and end the war inconclusively, but still gaining his entire kingdom.
All this require you to be able to land females. So my priority is getting the pagan reformation with ANY religion with Equality. Once you get it, it sticks to your realm regardless of your religion changes. I like to play Abbasid, and Zunist is my primary target. I can get the reformation without a war by offering vassalization or landing and murdering the ruler's son.
Sometimes you get to capture adult son, brother, or father of an enemy ruler before you get the pagan reformation. For those, I recommend inviting an African women as soon as the game starts, so that you can give him as a consort to her and then land him.