A great question. You are absolutely correct that a wave of post-modernism shook most of the humanities and social sciences beginning in the 70s. To understand the historical contexts that could have produced post-modernism (and made it so attractive across so many disciplines all at the same time) the ideology of "modernism" needs to be understood (as the movement "post-modernism" is reacting against).

Broadly, "modernism" has its roots in the Enlightenment project that elevates reason as a means to a better society. The developments of natural sciences in the 17th and 18th century (that human reason could be applied to understand the regularities and patterns of the natural world) eventually found traction as a way to understand the social world as well. Think of early economists like Adam Smith whose entire premise was that they could apply human reason to understand the patterns and regularities of society. Key to this way of thinking was that, given sufficient understanding of the patterns of human behavior and society, society could consequently be better arranged or organized into a superior form. For example, Adam Smith isn't just writing for the sake of describing economic behavior, but in order to argue for a particular organization of the economy based on the rationalism of his scientific pursuits.

Even the critics of "modern" society often where ultimately still following in this pattern of faith in rationalism as a means for social "progress". Consider Marx and other socialist thinkers - their contention was not that human reason couldn't be used to both understand and better organize society, but rather that the Classical Liberal paradigm was based on faulty reasoning and science. The fundamental conceit was still about an understanding of social progress.

The events of the early 20th century and the immediate post World War era effectively eroded the faith of many intellectuals in "modernity". In other words, it eroded their faith in human reason as a means towards social "progress". Consider when most of the most influential post-modern thinkers where born: Foucault in 1926, Derrida in 1930, Bourdieu in 1930, Barthes in 1915, Baudrillard in 1929, and so on. This places the adolescence of most of these thinkers squarely during WWII, and their early adulthood in the 1950s. By the time of the worldwide social movements in the 1960s looking to dismantle existing systems of social organization (colonialism, racism, etc.) these writers are penning their most influential works about the failures of modernism as an ideology and that resonates powerfully with intellectuals taking part in these social movements.

A huge number of events and social movements contributed to this erosion of faith in modernism, but WWII (and particularly the Holocaust) is one of the most cited influences. In many ways, the Holocaust was the culmination of the modernist project - the application of human reason towards the "progressive" organization of society through the industrialized and systematic destruction of human life. If that is the fruit of modernity, it makes sense that so many intellectuals of the same generation (growing up in the shadow of WWII) should all begin to reject the ideological foundation of modernity around the same time. While there were certainly precursors to this rejection of rationalism and modernity (Hume and other skeptics, or Nietzsche and Heidegger, for instance), the real sea change in academia as a whole ultimately rests on this generation of intellectuals born in the 1920s and 30s, and their disciples in the war-era and Baby Boom generations.

Another blow to faith in the ideology of modernism must certainly have been the end of European colonialism (and the critics of it both prior-to and after WWII, like Frantz Fanon, Ghandi and Nehru, etc.). The central tenant of European colonialism in the latter half of the 19th century and the early 20th century was that European nations were uniquely situated to bring the fruits of "modernity" (i.e. social engineering via rationalism) to the rest of the world. By the end of WWII, the choir of critics of European colonialism had pretty firmly demonstrated that the fruit of colonialism had largely not been the promised social progress, but rather continued exploitation.

In summary, the events of the mid-20th-century (including the Holocaust and the end of colonialism) eroded the faith of a whole generation of intellectuals in the value of rationalism and modernity as social projects. These intellectuals later laid the groundwork for post-modernism, which found a receptive audience in a world turned on its head by the social movements of the late 60s and which then resulted in the academic turn towards post-modernism in the 70's.

Edit: Typos.