So first, if it’s a fantasy novel you can pretty much make up any reason you want, and I honestly would recommend you make up something consistent with the world you’re trying to build, not try to be “historically accurate.”

But there are a lot of questions that would need to be answered to give specific scenarios. What analogous country are they from (England, Spain, Poland etc)? And what social class are they? And do they live in a town or the country? And does the husband practice a trade or a profession? And are they part of the dominant religion or a religious minority?

What is their social status?

Maybe he's an ambitious court bureaucrat and he's been given a cushy ambassador position.

Maybe they're merchants and they're going to the big city to open a store importing artisanal goods from their home town.

Maybe he's a soldier, and after a long campaign, they were granted a small homestead in the new territory.

Maybe he's a slave and she's a prostitute, and they're running away from their pimps and overseers.