Why has the field of psychology gone from being mostly men to mostly women?

Funny you should ask that question. In 2007 the American Psychology Association published an article called "The changing gender composition of paychology' in which they address this topic.

http://www.apa.org/monitor/jun07/changing.aspx

So I'll give a few inputs and then let you read the relatively short article.

It sounds a little trite to say that science fields are dominated by men, but this explains much. In the decades since Freud really blew the box open on the study of psychology, it has morphed into a much broader field and garnered tremendous opportunities for academic funding. At the same time it has been relegated to a 'social' rather than 'hard' science, like chemistry and biology, but more importantly physics and and applied mathematics.

Part of the explosive increase in the respectability of physics and mathematics (although they were always respected) is the pop culture influences of science fiction and the intersection of that science fiction on current research trends. The coorletation between what science fiction writers throughout the early twentieth to mid twentieth century imagined and built cultural awareness of, and the development of that technology by scientists is an interesting field on it's own. But as physics, mathematics and to a lesser degree chemistry drew much of the talented men away from psychology, a large vacuum was left for women to fill.

It's well known that women have been marginalized and excluded from most scientific endeavors with exceptions like Marie Curie and a lesser extent Ada Lovelace, so as a result, women have filled the void where men have opted to leave. This isn't to say women are somehow less scientific or less intelligent obviously, but we can see even today the call, or the demand, for greater gender diversity in the fields of robotics, of AI, of quantum computing and physics.

For all sorts of controversial reasons that aren't really suited for this sub, women in the last 40 years or so have increasingly gone to graduate school in far greater numbers, yet have failed to really break into the sciences, because of either overt or covert discrimination, because of an inherent gender bias, because of an apparent pay gap etc etc. As a result, women have filled the ranks of those fields where there doesnt exist such a bias or pay gap, such as psychology and the other social sciences.