Alright, here's some additional information...
Scotland wasn't absorbed, it basically broke up into its duchies. I had a valid heir, and my succession was primogeniture. No mods running. And my king inherited Scotland through birth.
I actually played it out twice... the first time I had been involved in a revolt, and I wondered if maybe I had hit "surrender" instead of "enforce demands". But, it did it again anyway. I checked the log, and it appears to have happened when my king inherited a barony. Not sure why that would make him unable to rule Scotland as well... Actually, right before inheriting it, I get the message that my king became unlanded. Here's the part of the log the first time around it happened:
And then the log from the second time:
Alas, I didn't save the game before overwriting it with a new one.