Unanimous, too, which was not expected.

NYTimes article explains: ... legal experts worry that upholding the Scottish ruling would set a troubling precedent. It would open the door, they said, to a form of judicial review that is widely accepted in the United States, which has a codified Constitution and a Supreme Court that actively interprets it.

Britain, by contrast, relies on an unwritten set of traditions and conventions that have treated a sovereign Parliament as the supreme law of the land. Once the courts venture into the political sphere and begin passing judgment on Parliament’s actions, legal analysts say, there is no going back.

https://www.nytimes.com/2019/09/24/world/europe/uk-supreme-c...