Sometimes change happens suddenly, but often we see that a supposedly sudden change has actually been building for a while. The dominos fall at once, but they've been set up over a long period of time.
Such changes build beneath the surface, and when they pop up, it is not a complete surprise. We recognize the source of the restless stirrings we couldn't name before. And then we know what we've been suspecting: we want to leave that job, or sell that house, or have that baby, or write that book.
As writers, we create something similar in our books, building the changes in characters so that they evolve naturally but satisfy dramatically. Readers want to see the dominos fall, the bonfire lit, the race run. We want to see everything in the book serve its purpose, fall into place or zoom into the air. This is what it all meant. This is where it was leading, we think, and for a moment, everything makes sense.