Selection effects: men and women may go into economics for different reasons; for example (as a bit of a caricature); men who want to get obscenely rich study economics to get into business, and women who want to get obscenely rich try to marry into money, and money-grabiness is correlated with pro-free market views.

Another version of this idea that I was going to post: is that conservative women are more likely to become stay-at-home moms and thus liberal women will be overrepresented in all jobs.