Part of it had to do with the Soweto Rising and the publicization of educational inequality; part of it also had to do with activist organizations pressing forward after winning battles over civil rights and Vietnam. So the acceleration in the late 1970s naturally connected to radical and soon even moderate students on US campuses who were more receptive to the basic injustice of the situation as they saw it. Some of this built on organization that already existed; at Michigan State, where I was in school (albeit much later), the core of this movement was the East Lansing SALC (Southern African Liberation Council/Committee) which actually formed around 1972 or 1973. In the wake of the Soweto rising from 1976 on they had a lot more cachet to push for divestment, and much greater interest from the students themselves--first for the city, then for the University. Swarthmore's Nonviolent Action Database has a pretty good write-up here. Michigan State was among the very first--starting later but divesting slightly sooner than Wisconsin-Madison, which also has a very good writeup, so it's an interesting case. It's also home today to the African Activist Archive, which is worth checking out in its own right.