The same question would always pop up in my mind: "Why? What happened with their commitment?"
Until recently I concluded that the only answer is... well, people change.
Someone might get new insight that lead to new life goals, and the spouse failed to join her new life journey. Someone might get spiritual experience that make her more religious, and the spouse didn't agree with her new perspective.
And she might meet someone else who understand her better than the spouse.
The world is always changing, so are the people.
People always have the choice to stick to their commitments and communicate the changes to their spouse, or to move on to the next journey and leave the spouse behind.