It's hard to say what happened without more details.
Here are a few possibilities that I can brainstorm off the top of my head:
- Maybe the English uncles became bishops which disqualified them from succession
- Maybe the English uncles got blinded or castrated (unlikely, but technically possible)
- Maybe the English uncles are the result of a matrilineal marriage. And a recent succession law to Agnatic excludes them from succession (since inheritance under Agnatic does not go through or to a female).
- Maybe High Crown Authority is preventing the inheritance. Could be an issue if the English uncles are landed or heirs to titles that are outside the realm.
- Change to Elective succession law could easily result in an heir of a different dynasty
- Change to Primogeniture, Gavelkind, or Ultimogeniture could also result in a surprising result, if an unexpected matrilineal marriage messes things up
Additionally: In 1066, the throne of England has more pretenders than grains of sand on a beach. The king tends to get kicked off the throne a lot, especially if William or Harald succeed in their conquest.
Not only that, but someone (or multiple people) could have had sons. For the likely succession laws, Agnatic-Cognatic Primogeniture, the king having a son will default succession to him. If the current heir has a son then that son will be added to the line of succession after all the king's sons. Like so:
King has one sons, 3 brothers, and 2 daughters. The succession goes:
- Son
- Older Daughter
- Younger Daughter
- Oldest Brother
- Oldest Brother's Sons
- Oldest Brother's Daughters
- Middle Brother
- Middle Brother's Sons
- Middle Brother's Daughters
- Youngest Brother
- Youngest Brother's Sons
- Youngest Brother's Daughters
- Sisters
- Sisters' Sons
- Sisters' Daughters
- Uncles
- Uncles' Sons
- Uncles' Daughters
- Aunts
- Aunts' Sons
- Aunts' Daughters
So as you can see, if your daughter is matrilineally married to the Youngest Brother, then you have to go through all the King's sons, daughters, the other brothers and their sons and daughters all before the Kingdom passes to the Youngest Brother. Even if he was say... third in line before, if his older brother had a couple of sons, and the king had a couple of daughters, then suddenly the Younger Brother is back to being 10th in line.
It gets even more convoluted when dealing with multiple generations -- The king's grandson will inherit before the king's brother will, or even the second son. The king's brother's grandson will inherit before the king's youngest brother.
The heir being of a different dynasty is very easy to rationalize -- One of the king's daughters who is dead could have had a son before she died.