That was stated in the article: Organic shift through curriculum redesign and change in teaching methods. Staffing probably also played a part, although that wasn't mentioned in the article beyond Klawe.

Note that students at Harvey Mudd are not admitted to any particular major or program, they are admitted to the college in general and don't declare a major until Sophomore year. An increase from 10% to 55% is at least partially a result of more female students taking interest in the subject, not a result of admissions. Matriculation of female students to the college as a whole also rose over the past decade, but that went from ~35% to ~50%, so it accounts for less than half of the change in CS.

(Disclosure: Harvey Mudd alumn, class of 2008)

Can you elaborate on the change in teaching methods? I don't understand how CS101 could be gendered in a way that Math or Physics isn't.

(Mudd alumni, class of 2011)

One big thing is splitting the intro CS course into different sections based in previous experience. Folks with a ton of experience end up one section, and those who are new to the subject in another.

The two sections cover the same material, but with different lecture styles, and people don't end up discouraged because their labmates cruise through exercises that they find challenging.

This isn't explicitly gendered, but pre-college exposure to CS and programming could well be correlated with gender...

The course website is here, and shows the Gold / Black sections: https://www.cs.hmc.edu/twiki/bin/view/CS5

The acceptance rate for males is 9%. For females it is 21%.

This will likely be true in the short term. But experience with other professions shows that the phenomenon will likely be transient. For example, despite the fact that law school is gated by an entrance exam where men outperform women by about the same margin as they do on the SAT Math,[1] law schools have even gender ratios with similar admissions chances for each gender. That wasn't necessarily true back when law schools (and med schools) took affirmative measures to equalize gender ratios, but the very process of equalization removed barriers keeping women from considering the field.

Fundamentally, most sensible people do not want to create additional hassles in their lives by choosing to become a minority. If life in Bangladesh had been as great as life in the U.S.A., my parents certainly wouldn't have moved to a place where they looked different from everyone around them. When women consider going into male-dominated fields, that's essentially what they're signing up for. That dissuades a lot of candidates who would otherwise be promising.[2]

[1] While men and women perform similarly on average on both tests, there are significantly more men who score in the top 1% on each test (for whatever reason). In the numbers-driven world of law school or med school admissions, that factor is outweighed by the fact that women tend to have better GPAs and as a result have similar admissions composite scores.

[2] The same is of course true for men considering women-dominated fields. There are many men who would be phenomenal teachers, nurses, child-care workers, which are solid jobs with good pay, who very reasonably do not want to put up with the hassles and skepticism that would come with being a man in a women-dominated field.