I don't think it's about absolute numbers, women did not stop coding. It's just more men starting to code, which makes the share of women smaller.
http://i111.photobucket.com/albums/n134/im_thatoneguy/CSDegr...
Interesting that there was a dip of both males and females around 1984. So I'm not so sure of the statement above anymore. However, around the year 2000 there's this peak where males do outnumber females by a great deal (even if the number of females joining also increased).
If all these graphs are accurate I'd wonder more about the Y2K rather than 1984...