> drop off was nearly identical for men
> women did seem to drop off more
These two statements cannot be both true at the same time.
Author notices that number of women in CS significantly went down in 1984 and searches for reasons. According to the graph provided in the parent comment it was actually CS as a whole which went down. Both lines on the graph are awfully similar, and one can assume that there wasn't any sudden catastrophe for women in CS at that time.
When analyzing 'What Happened To Women In Computer Science?' the absolute number is not important, but the ration of women in CS and when that went down (and why).