Headcanon that Isabela is the sort of person who reads all kinds of fanfiction romance novels and complains loudly about how inaccurate they are. “Of course you smooth over some of details since it’s fantasy, but you simply can’t put that there, you are not doing that without any sort of preparation, and by this point, neither of you is going to be smelling like anything besides sweat.” And most of the others are like “Maker I don’t want to hear this” but Varric just sort of nods and listens patiently, because even though he is super asexual, he’s a writer, and that’s what writers do; listen and absorb information just in case it shows up in a story.
And maybe one night the two of them have had a bit too much to drink, and they’re joking about how terrible sex scenes in mainstream romance are, Isabela dares Varric to write one (because Donnen Brennakovic is also super asexual, and there’s nothing like that in Hard in Hightown). So he does, and later, he’s coming up on a deadline because his publisher wants a short stand-alone piece from his as part of an anthology or something, like maybe one of their other writers bailed (or died; it is Kirkwall) and Varric is a last-minute substitution. And he’s really irritated with his publisher for springing this on him last minute, not to mention how busy he is running around with Hawke, so he’s like “ok fine have this sexy trash I wrote while I was drunk it’s all I have lying around right now” and his publisher is like “we’ll accept this under the condition that you expand it into a full serial by the end of the year” and Varric is like “well shit, I don’t have time to finish anything else now, and I need the royalties so I can afford to keep paying all the bribes, so at least this will buy me some time.”
So that’s how he ends up contractually obligated to write Swords and Shields, even though he’s like “I have no clue what I’m doing here, Andraste take the reins.”