How is the head of dynasty decided?

Question

My character just died. He was the head of his dynasty. But when he died, I was surprised that my new character (his son) was no longer head of the dynasty. Instead, the head of a cadet branch which split off from his house became the new dynasty head.

Here are some more details about the structure of the dynasty. The founding house was already extinct at the start of the game, but he was the head of the first and only cadet branch (the one with the non-divided coat of arm). Over the course of the game, two cadet branches split off from that house. Now the hierarchy looks like this:

Founding House (extinct since the beginning of the game)
    - My House (used to be head, but isn't anymore)
         - Cadet Branch A (now the head)
         - Cadet Branch B

Why did that happen? And is there some way to get back the head of dynasty position to the player-character? My goal in this playthrough is to grow my dynasty renown, so I really would want to keep playing as the dynasty head.

Answer 1

I stumbled upon this entry from the CKIII FAQ posted by Paradox themselves:

Q: I understand that the head of the most powerful house head becomes the head of the dynasty, but what determines who is the most powerful house head? Which factors are the most relevant?

A: Military strength is the main factor

But it appears that when the head of the dynasty dies, then the new head is determined before the titles of the old head get passed on the heirs. As your primary heir will usually not have that many titles and vassals at that point, the head of the house title will often pass on to a different ruler of your dynasty.

Fortunately, that's only temporary. A couple months later, the game will reevaluate the military strength of all dynasty members, see that your character now has all those troops from the inherited titles and vassals, and give the head position to the now strongest character (you).

Answer 2

Another thing to consider here is that only house heads can become dynasty heads. House Head is directly inherited by the primary heir (i.e. usually your oldest son/child), even if you use elective succession to make another child your heir.

So, when you die, check whether your heir is the house head, if not, you must make a new cadet branch to become eligible to become dynasty head again.