There seems to be some huge overall spike that coincides with the official end to the Vietnam War (end of U.S. military involvement, that is), Watergate, and the election of Jimmy Carter.
It was in the late 1960s that administrators discovered “teaching evaluations.” That’s when the average grades start to rise dramatically. Tenure-track and temporary instructors are very vulnerable to this supposed “evidence” that they aren’t teaching well—but students who expect a good grade in the course aren’t as critical when filling out the evaluations.