Narratives are Journeys
A novel is a work of art that tells a story. It is a journey (perhaps we could talk about a Grand Journey, being the sum of several journeys), taking the protagonist - and vicariously the reader - from point A to point D.
However, and this is the crucial point, journeys in narratives should not usually be linear. You don't go from point A to point D simply passing by B and C. Indeed many inexperienced writers (particularly of genre fiction) fall for that trap.
Think of it like this: you leave your house on your way to visiting your friend. You could conceivably go straight there, put you could also pass by the park or the lake, the liquor store or the supermarket. This
a) makes the journey more interesting
b) opens up the opportunity for choices
It's exactly the same thing with narratives. You, the author, have full freedom to choose the path from point A to point D.
With one crucial detail, which takes us to...
Inevitability in Narrative
Yes, you can choose anything you want leaving your house on your way to your friend. You can choose to go to the liquor store. However, this choice limits your choices from that point on. In our example it means that visiting the liquor store you don't have enough money for the supermarket. This entails some inevitable repercussions for the journey.
For instance, it means you can watch the game with your friend while drinking beer, but you can't have pizza. If you passed by the park on your way to your friend's you could talk about the interesting shrubs you saw. But you can't talk about the ducks swimming in the lake, because you didn't pass by that location.
This is a very simple example, for sake of clarity, but in a full-fledged narrative the author is dealing with literally countless such divergence points. We don't consciously think most of them, so the work has to be made subconsciously.
Inevitability in Endings
The most crucial aspect of inevitability in narrative is the ending. The reason is that that's where it all has to come together. And so, an inevitable narrative is one where the ending feels simply like a sum of everything that has occurred by that point. In our example, an inevitable ending is you watching the game with your friend drinking beer. Again, it comes off as self-obvious, but that's only because the example is very simple (practically binary).
A strong marker of an ending that is not inevitable is if you don't know what to do with it. And perhaps the strongest marker of avoidable endings is if you need to come up with a fanciful solution to justify it.
- "I'm hungry man, you didn't get any pizza"
- "Well, whaddaya know! I accidentally forgot I had these burgers in my backpack"
Inevitable endings/narratives are not always easy, but they're the only ones that don't come off as cheap.
Source and Further Reading