if this is biochemical, what is the common thread? What do all of our spouses have in common?
I am not sure that all of our spouses have to have the same biochemical/developmental/ physiological cause.
Let's take something like heart disease. There are many factors that cause heart attack and stroke. Your risk increases with the number of risk factors you have but people can have different factors yet still end up in cardiac arrest.
So, cardiovascular disease can be due to obesity, smoking, genetic history, high cholesterol, high blood pressure, diabetes, stress, lack of physical activity, unhealthy diets and/or harmful use of alcohol. Some young women who take birth control pills will have a stroke.
So, unlike infectious diseases, that have a specific cause, other illness can be due to many factors. We have all heard of an athlete who has a heart attack, often due to heart malformations that were never detected.
So too with our spouses. There have been several possible reasons for their crisis but there is no definitive test to determine what caused such a break in their personality.