The cited book provides very interesting background:

> Admissible grounds varied from state to state. [...] In the other states, the common grounds were adultery, desertion, and cruelty; but there were all sorts of state idiosyncrasies. [...] Cruelty became the grounds of choice in the twentieth century; it overtook adultery in 1922, and in 1950 accounted for almost three-fifths of all divorces. Most states recognized cruelty as a valid reason for divorce—New York was a prominent exception.

> What accounts for this outbreak of marital cruelty? Nothing. It was, in fact, an outbreak of collusion. Most "cruelty" cases were uncontested. The plaintiff (usually the wife) filed for divorce. The husband made no defense. Divorce was granted, by default. Collusive divorce had become common in the late nineteenth century; in the twentieth century, it was absolutely pervasive. In legal theory, a collusive divorce was void. Husband and wife had no right to agree to split. In practice, collusion was the rule, not the exception; and the judges all knew it. Their (implicit) motto was: don't ask, don't tell.

> The precise form of collusion did vary from state to state. It mirrored the state statute; it was, in a sense, "cheating in the shadow of the law". In California, as in most states, cruelty was the courtroom favorite. In case after case after case, the wife complained that her husband cursed her and hit her, and made her life miserable. [...] In 1921 in San Francisco, the wife was plaintiff in 70 percent of divorce cases; and she alleged cruelty in 40 percent of the cases. By 1950 the percentage of accusations of cruelty had reached 70 percent.

> In New York divorce was available, practically speaking, only for adultery. This was an extreme situation; but any and all attempts to amend the law ended in shipwreck in the legislature. The demand for divorce, however, was as strong in New York as it was elsewhere. One end-run around divorce was annulment. Annulment is a declaration that a marriage never was valid, because of some kind of fraud or other impediment; in most states, annulment was a rare beast—usually fewer than 4 percent of all dissolutions of marriage. But New York was an annulment Mecca. By 1950 there were ten counties in New York which granted more annulments than divorces; and for the state as a whole, there were two-thirds as many annulments as decrees of divorce.

> New York also developed a weird form of collusive adultery—one might even call it soft-core adultery. A man would check into a hotel, a woman (usually a blonde) would appear, together with a photographer; the photographer would take pictures of the couple, in pajamas or underwear or even naked; the woman would get her fifty-dollar fee, and lo and behold! here was evidence of adultery. The flavor of this charade is neatly captured in the title of a magazine article from 1934: "I Was the Unknown Blonde in 100 New York Divorces".

Taking the above at face value, we have strong evidence that lots of people went to major lengths to establish a legal pretext for divorce. Cruelty allegations going from 40% to 70% in one city, and apparently making similar changes elsewhere, is certainly very suspicious—does it seem likely that husbands' behavior actually changed in that way? Allegation statistics varying significantly from state to state would also be rather suspicious; the book alludes to that but doesn't give details. The cited book itself has citations for several of the numbers and the magazine article, which one could chase down if one wants more detail and confirmation.