I'd guess that Norway's last settlement was captured - which is quite surprising unless you did it since CastleTown or whatever it's called is usually quite safe on its little island with a ludicrously large Norwegian army sat around it.

I'd guess that Norway's last settlement was captured

Norway had all its starting provinces EXCEPT castle town, which was taken by Wales.

Well if Castle Town was taken then that means that Prince Magnus died, so Haakon would have had no heirs. I imagine that, because he is a character with special circumstances as he enters the game later on, he might not properly count as a Norweigan family member. Weird though as you would at least think he could've had the chance to adopt before he died, allowing the Norweigans to live on.

Kingdoms is notorious for the fact that certain factions can be killed off pretty early on, despite still having most of their settlements. On several occasions after Haakon has died I've diverted all my resources to taking Castle Town where Magnus is, allowing me to take the Western Isles in rebel form with weak garrisons. It's strange that an AI faction took Castle Town though. Occasionally Wales will land an army but they never seem to attack, giving the Norweigans a chance to pump out several full stacks until the size of the Welsh army becomes pretty irrelevant.

In the Crusades campaign as the Kingdom of Jerusalem it is possible to make a suicidal march into Egypt, take Cairo and then watch all the Egyptian provinces turn rebel. Saladin never seems to leave Cairo. While initially fun though, once Egypt is gone it makes for a pretty easy campaign as when a faction turns rebel they seem to lose most of their army and garrisons. And obviously they beome more passive instead of actively trying to stop you from marching through their lands.