The shift itself happened gradually. The first thing to go was divorce by mutual consent. One had to have a “serious” reason like adultery, incest, impotence, or the desire to take religious vows in order to end a marriage. Then came the restrictions on remarriage after divorce. A big part of Lothar II’s problem was that he needed a divorce that would allow him to marry again. By the 10th century, only incest, non-consummation, prior marriage, or prior religious vows were valid reasons to end a marriage.
On a very general level, one could say that there were two sides in this: the Church and the nobility. The Church tended to push for the idea that marriage was permanent and entirely under its jurisdiction. The nobility, on the other hand, wanted to keep their right to divorce as suited their political needs.**
[Carlo Calvo, 10th century, Psalter, source: Wikimedia Commons]
*This doesn’t mean there weren’t obstacles to it, but these lay more in the will of one’s family and one’s spouse rather than the culture and the Church.
** We almost never hear about the opinions of anyone outside these groups in large part because this was such a political debate. This possibly also meant that the lower classes in any given area were more likely the freedom to divorce for longer, since those most strongly pushing for reform were less interested in them.
Coontz, Stephanie. Marriage, a HIstory: How Love Conquered Marriage. New York: Penguin, 2005.
Stafford, Pauline. Queens, Concubines, and Dowagers: the King's Wife in the Early Middle Ages. Athens, Georgia: University of Georgia Press, 1983.