My guess is it's rare when someone doesn't see it coming.
They may ignore it or refuse to believe it.
Very similar to the old Gladys Knight & The Pips song:
"Neither One of US" where two people realize it's over but neither has said it out loud.
There are some people that never "legally divorce" but are "emotionally divorced". They may live in separate areas of the house and each person does their own thing including seeing other people.
However in most instances there is usually a "deal breaker" committed in the eyes of one person; (cheating, repeatedly verbal/physical abuse, drug/alcohol addiction, constant arguing, physical and emotional neglect..)
Eventually one person realizes they can't live the rest of their life this way. Once they realize they can't "change" another person their only other options are accept them as they are or move on. Life is short.
Having said that I believe the top two reasons for divorce are:
1. Choosing the wrong mate
This often happens when we have not figured out who we are and what we want in a mate for life (before) we pursue relationships. It's the equivalent of going shopping without a list!
They have no real mate selection criteria.
They allow "impulsive connections" and "happenstance" to dictate their relationship choices. Never separate your mind from your heart when making relationship decisions. The purpose of the mind is to protect the heart.
2. Getting married for the wrong reasons
An ultimatum was given, an unplanned pregnancy, had an age goal to be married by, all of their friends were married, just got tired of dating and being single, someone was about to be deployed on military duty and wanted to have someone waiting for them, the perspective spouse has money, fame, or connections, had been "a couple" for years and decided (why not) take it to the next step, fear of never being proposed to again...etc
A marriage based upon circumstances rather than love is likely to fail.
The goal is to find someone who shares your same values, wants the same things for the marriage that you do, naturally agrees with you on how to obtain those things, and last but not least there is a mutual depth of love and desire for one another.
Like attracts like and opposites attract divorce attorneys!
Staying together for the wrong reasons is just as bad as getting married for the wrong reasons.
"Never love anyone who treats you like you're ordinary."
- Oscar Wilde