I have just discovered that there are 341 works in the ao3 tag “Canadian Shack”….. as a Canadian idek what to say. I can’t believe that’s like…. a trope
Oh! Oh! I know all about this!
As a Canadian, you are presumably familiar with the1990s buddy-cop TV show Due South, about a RCMP constable named Benton Fraser, who has come to Chicago on the trail of the killers of his father and for reasons that do not need exploring at this juncture has remained, attached as liaison with the Canadian consulate. Anyway, it's a buddy cop show about Fraser the Mountie and his Chicago cop partner Ray, who is a different person depending on which season of the show you are watching.
So dS fandom shipped Fraser/Ray, Fraser/Ray (the other Ray) -- and, yes, the ship wars were legendary -- and, for the truly daring, Ray/Ray. The series finale "The Call of the Wild" ended with Fraser and one of the Rays in the Northwest Territories, sledding off into the sunset together on an adventure. So naturally fandom wrote a lot of fic about what this adventure consisted of and how it very possibly involved Fraser and Ray together in a shack, in Canada. (Fraser does actually at one point have a cabin somewhere around there in canon, IIRC, so the fanon that he might at some point once again end up in a cabin somewhere in the Canadian north isn't actually coming out of nowhere.)
Anyway, so fic about Fraser and one of the Rays in a Canadian shack became a popular premise for a whole lot of post-series fic, and then the thing that happened was that a bunch of Due South authors looked around at the massive amounts of Canadian Shack stories in their fandom and thought, "Hey! What if we had a multi-fandom challenge where all of our other fandoms ended up in a Canadian shack too?" and that led to 101 Ways To End Up In A Canadian Shack, which as the name suggests is 101 ficlets by various writers in various fandoms (many of which are not set in Canada, or in North America, or even on Earth) all running with the Canadian Shack trope from Due South.
And then, as happens with fanon, other fans who might not have even been familiar with the Due South origin picked up the trope and ran with it because who doesn't want to stick their OTP in a shack together because hooray forced-proximity tropes, and it became a thing that took on a life of its own, and that's where we are today.
So, yeah. It's one of those tropes that maybe makes a little more sense in its originating fandom but that everyone else has subsequently adopted in their own fandoms -- like the daemons from His Dark Materials or the psychic wolves from the Iskryne series, or, for a certain value of "making sense in its original fandom," Sentinel/Guide tropes (which are based on a popular genre of very intricate AUs in Sentinel fandom and doesn't really have much basis in canon).
...I was wondering about this. :)