All of the above.
Heck, I almost lost my job after the bomb drop, from the stress and the lack of sleep.
Some of it probably also has to do with "discovering themselves"; aside from their marriage, their job is probably the other major thing they think is contributing to their unhappiness. So they may think they want to change careers, or try something more creative or "fulfilling". Maybe they want to live closer to an OP, or farther away from the people they think are making them unhappy.
Me: 45, Her: 40. Married 16 1/2 years, together(-ish) 20.
Status: BD 8/25/09, she moved out 8/28/10. No talk of D.
Every day is another chance to get it right.
http://www.vachss.com/mission/behavior.html"Counting days won't buy us years" —Wings by HAERTS
"Forgiveness means giving up all hope for a better past." —Lily Tomlin
"When we commit to our lovers, we implicitly promise to forgive them. There is no other way we can live with someone for better or worse or until death do us part." —Dr. Frederic Luskin