Americans invented it DAMMIT!!!! Instead of national Brewing pride, we divide ourselves.
Here's where I see it, and let me be clear, I in no way claim to represent Abram or the other folks who coined the CDA moniker... however:
Some dudes in Oregon realized they really liked hoppy, slightly roasted beers. The versions that came to mind to them as first existing were Rogue's Skullsplitter and Phillips's Black Toque (since renamed Skookum Cascadian
Brown Ale but that's a whole 'nuther story...)
They started brewing it themselves in their homebrew setups and encouraging their local brewers to make the style as well.
They are also fans of brewing organic beer.
They drink some Fish Tale Organic IPA, which is labeled as "Brewed in the Republic of Cascadia." They think that sounds kinda awesome. They notice that Victoria, Olympia, and Newport are all within the borders of this fictional nation, and coin the term "Cascadian Dark Ale."
Hopworks Urban Brewery in Portland releases the Secession Cascadian Dark Ale. It is also labeled in merchandise, on the bottles, and in the pub as a "Black IPA." There is also, however, prominent maps of the Republic of Cascadia and images of the "Doug Fir" Flag, which is the flag of the Republic. (You can purchase a flag for yourself from a small flag company in Salem, but I digress...).
It's clear to most that Phillips and Rogue independently developed very similar recipes for dark, hoppy beers. I think it's also clear that these beers were developed independently of brewers in Vermont, Texas, or San Diego. Hell, this style has probably been independently developed by homebrewers all over the world... either innovative alchemists who took chances or screw-ups who accidentally tossed some Carafa into the mash tun.
It's a crazy thing, no doubt. But keep in mind, The Republic of Cascadia is FICTIONAL. There was a real effort at one time to re-name Imperial IPAs "San Diego Pale Ale," and this is part of what pissed off CDA-boosters. Isn't using a fictional, hilarious name like Cascadia better than actually naming a particular city or county?
Part of what's awesome about Cascadia is that it jokingly refers to breaking off states and provinces from two different countries, basically because all of the new citizens would share the same tastes in rain gear. Cascadia evokes images of tall trees, clear water, and Sasquatch... lots of Sasquatch.
Enjoy it. It's like having a "Narnian Strong Ale" or an "Arcadian Mild."
So relax, kick back, and have a homebrewed "Mordorian Stout" or "Oceanic Bock."