Watchtower invented the entire blueprint. The original "Family book" added "Spiritual endangerment" as a reason for "scriptural" divorce and re-marriage.
What is spiritual endangerment? Well, it was a nebulous excuse...that became whatever people and a couple of elders decided it was.
Perhaps someone yelled. Missed meetings. Rolled their eyes about "new light". Want sex a particular way...or frequency? Didn't earn enough money? Worked too much or too little? Was not a spiritual leader?
A divorce or separation would follow. The "endangerer" eventually got lonely or horny, dated, was simply alone for a few hours with someone, or had sex...and voila! The one claiming they had been " endangered" became "scripturally free" with the congregation elders and service departments' blessings to remarry.