How it came about

Northanger Abbey is not the first novel Austen started and may not even be the first one she finished. It is the first completed text that we still have. She started both Elinor and Marianne and First Impressions before this.

How did this novel come about?

This isn't even a theory, just a speculation. We know that Austen liked to do burlesques of novels for her family. Perhaps, having started and having been frustrated with the results of the other two novels, she fell back on writing a burlesque because it was a comfortable thing she knew she could do well.

I read years ago that the creators of thirtysomething began their proposal for the show as a joke, as a spoof. At one point, having put an awful lot of work into the proposal it hit the creators that this might get taken seriously and they might end up having to actually make the show. Faced with that possibility, they decided to do a proper proposal.

My speculative thought is that Jane Austen had tried two novels and was unhappy with the results so she fell back on her strength, dashing off burlesques. But as she was doing it, and perhaps helped by her two false starts, Austen noticed that there was rather more to this burlesque than there had been with the others. And then she realized that here, without meaning to, she'd had her break through.

As evidence, I note the attitudes. The book starts off very self-mocking and then becomes progressively less so.