Another thought: compared to other technical careers, programming careers are uncredentialed. No legal or even social prerequisites other than skill.
If you want to be a doctor, you must apply to college, be accepted to college, matriculate, graduate with a degree in sciences. Then you must take the MCAT, apply to medical schools, be accepted to a medical school, graduate, apply to a residency, complete the residency, and become board certified.
And then you are an entry level doctor, licensed to practice in a single state, with no job, no patients, no practice, and no insurance network contracts.
If any of those steps are sexist/racist, a woman or minority is less likely to make it past that step.
Programming doesn’t have as many gatekeepers. That’s good for people who have been kept out of things.
The drop in women's representation in computing corresponds to the elimination of overtly sexist policies in other fields. E.g. medical schools removed gender quotas (maximum not minimum quotas, sometimes as low as 10%). Mary Ann Wilkes initially wanted to be a lawyer, but changed to computing when she encountered explicit sexism in law firms. After the expansion of women's right, women in her position were able to pursue their original passions.
I doubt it boils down to any one cause, but I think it's crucial not to ignore women's agency in making career decisions.