When talking about demographics and college degrees, i think popular culture is certainly relevant. We are talking about high school and college freshmen discovering themselves. Thus, i'd like to point out that 1984 is the same year the film Revenge of the Nerds came out.
Thus, i'll throw out the hypothesis that, since 80's popular culture was a very regressive era in terms of anti-intellectualism, desire to enter STEM probably took a serious hit at the time in general, much more so with women. That is not to say that previous generations were much better, but gone were the days where the space race inspired tons of kids to pursue STEM education regardless of gender.