by hostility, it seems that you actually mean "Why do blacks and women represent a minority in computer science?"
Computer science has always been an inbetween between engineering and hard sciences--because of that many laymen were employed to do menial computer tasks early in history. This was, perhaps , one of the first chances for laymen to jump onto a scientific industry which turned out to be semi-permanent.
Since we know that education paved the way for the empowerment of minorities, could it be that the statistics that you're referencing are really just moving as a function of women becoming wider dispersed and accepted within all sciences, rather than just the engineering ones?
Perhaps the statistical anomaly that you cite is really a glimmer of hope for women. Computer science is not the only field that readily accepts them now-a-days, and perhaps those numbers actually indicate a migration across a wider array of science and engineering fields that were not at one time available for them, rather than a disappearance of women all together in STEM.