Divorce is often caused by subtle things that one partner does not even realize are breaking the relationship apart. It's not always something obvious like domestic violence or an affair or substance abuse.
For instance, when two people have very different expectations for the marriage, that can leave one of them feeling neglected. Perhaps one partner thinks that it is the other person's job to financially support them. That person has no idea that the other spouse does not want to work. When the couple runs into issues with employment and budgeting, whose fault is it? Money problems cause a lot of stress in marriages, and they can come from the fact that both people did not view money the same way from the very beginning.
Emotional expectations are an issue, as well. One person may want words of affirmation; they need to be told how important they are to the other person. The other person does not vocalize those things very well and is better at showing their affection by working hard, making money, cleaning the house and helping out with the kids. Those are all good things, but they can still lead to resentment when the other spouse does not feel loved. Meanwhile, the spouse who is better at showing than telling does not even know there is a problem and feels like the relationship is as strong as ever.
These types of issues show why it sometimes feels like divorce comes out of the blue. If you get blindsided when your spouse files for divorce, make sure you know about all of the legal rights you have.