I didn't really live through that era, but my suspicion is that more well-paid roles (especially in management and health care) opened for women during that era, and so they decided to work in different fields more in accordance with their own preferences. Hence the decline from women in computing. Basically, I believe it is in line with the gender-equality paradox.

It can also be the case that the whole thing is a dynamical system (for example driven by desire to work in a field which already has majority in the same gender), and it somewhat chaotically "oscillates" around the equilibrium. So there really is not an causal explanation as such.