Divorce did not happen in The Middle Ages as one might think. Often, in cases of Royalty and Nobility, their marriage ceremonies were conducted under the auspices of The Church. Having been preformed on “Holy Ground” - this constituted a Sacrament (which COULD NOT be broken). Often, (at least prior to England's King Henry VIII) – such needs for divorce were NOT entertained. Rape, adultery and incest (although crimes) – were NOT considered grounds for divorce! However, a common practice among Kings (especially young ones), was divorce on grounds of “infertility”.
A King required a Male Heir to govern after his death. The Queen's primary duty, was to produce one. Often, accomplished (after many Girls) – a King would consider her duties “completed” and he would set about the task of preparing his new Heir to rule. If a Queen could NOT produce an Heir – it was considered “valid grounds” for divorce. The Catholic Church did NOT grant such dis-unions with ease. Most of the time MUCH political pressure (and bribes) were needed. In Commoner cases, it was similar. Often, Commoner's did not marry inside a Church (preferring to do so at the Church Door or Gate). This made it “legal” but NOT a “sacrament”. Also, many Commoners moved to “Civil ceremonies” that did NOT involve a Priest (rather a Magistrate or Justice of The Peace). As a “lega; Contract” - they found it would easier to “break” such! Many Nations today still have a type of “Common Law Marriage” system. This system stated that a Couple who “cohabitate” for a certain period as “husband and wife” - are so legally identified as such.
In some cases, “mental instability” was cited for divorce. Again, a woman in The Middle Ages had very little “marital rights”. If her husband could establish his wife was incapable of functioning as a proper “wife” was expected – he could sue for divorce (and often get it!). In other “darker” Cases, dissatisfied Kings and Noblemen resorted to poison or “accident” to rid themselves of their “problem”. Many time, Trials were held – and quickly dismissed for “lack of evidence”. Later, such “assassinations” were taken more seriously (which could by this time ACTUALLY mean an execution of murder in some Kingdoms, regardless of Title.)