How do some institutions attract so many female computer scientists?

Carnegie Mellon gives an answer on their website:

The steady climb of women enrolling in these fields at Carnegie Mellon highlights a combination of factors: a strong commitment by leaders at the university, college and department levels; influential pipeline programs for middle- and high-school students; targeted recruitment; closer scrutiny of applications; support and mentorship programs; and attention to diversifying the faculty.

A second, speculative answer: Carnegie Mellon is wealthy. Computer science is where they invest the most. If they want to attract more women to their computer science department, they can use their money to do it. Many of the factors they list cost money. They may be decreasing the diversity of other universities.

We are a smaller German University of Applied Science and most of our courses have a certain application field in addition to computer science (usually about 70% computer science, 30% application related). The application fields "media" and "medicine" seem to motivate many female students to start studying, even if they later decide to work in the automotive industry or somewhere else.

Furthermore, our university strongly invests in diversity actions, we are trying not to discriminate anyone, and we are trying to promote female students e.g. as student assistants / tutors (which is not hard, since we are usually looking for the best, and most times we are having more female top students than male ones). But for the younger female students it is good to see that others succeeded or are very successful.

Once you crossed the 30% mark, the feeling is that it's not "special" or "strange" if you are female.

One point where we have to improve is the number of female faculty members, but this takes time.

But still, motivating more students to study computer science is a high priority!

At the risk of sounding like a broken record:

I'm especially interested in answers from female computer scientists at CMU.

Well, go ask (some of) them directly - don't expect them to stumble onto this page...

The staff directory is available; you'll need to figure out which ones are women by the photos and the names though, and pick out a few people to contact. Or you could start by contacting the Dean or whatever CMU call the head of the School of Computer Science.

The School of Computer Science at CMU also has a dedicated Women @ SCS website, which you should check out, both for immediately-relevant material and for names of women faculty and management personnel which you could contact directly (i.e. put those at the top of your list of people to contact). Thanks to @user3067860 for noticing that site!

PS - As commenters suggest, you can't just start asking specific questions with long answers right away, you need to make sure whoever it is you contacted is willing to converse regarding these issues.