I posted this in the thread but everyone else seems to think the big jump in A's was like a precipitous cultural decline.

Actually, it was the escalation of the Vietnam War combined with a conscription policy in which lower-graded students would lose their draft deferments. https://twitter.com/annakhachiyan/status/1265030204227026944

Professors didn't want to be the reason their students went to Vietnam, so they started giving out a lot more good grades.

The war went on long enough to permanently change the meanings of the grades, it seems.

Obviously the merits of this kind of protest are debatable. I learned about it from an infamous old college debate case ( @alanontheright); we argued the professors were doing wrong by favoring their students in this way.

And over time there have presumably been other influences on grade inflation.

But I believe the big spike in the chart comes from professors acting against the Vietnam draft.