Interesting data via Professor Schwitzgebel.  I can imagine a couple of factors that have contributed to this trend, which seems to have gotten under way in the early part of the last decade.   

First, starting about 15 yeras ago, many PhD programs made a concerted effort to increase the recruitment of female students, with good results it would appear.  Second, sexual harassment, long the scandal of the profession, came out into the open as an issue that had to be addressed.  Many now relatively senior female faculty have told me that when I first posted to warn prospective students about "sexual predator" faculty in 2009, it was the first time a relatively senior male figure had made any very public statement about this problem they knew all too well.  I think the now defunct Feminist Philosophers blog, and it's affiliated sites, like What It's Like to be a Woman in Philosophy, helped bring further attention to this issue, wiht the result, I think, that by the middle of the last decade there was  robust consensus that "enough was enough" with sexual misconduct by faculty towards students.  I have not done a systematic survey, but the senior female philosophers I have asked all agreed that there was much less sexual harassment in academic philosophy than there was in 2009, which is very good to hear.  This would, again, explain why more women have found they can pursue professional careers in academic philosophy without incurring the costs of harassment that, in the past, drove many out.

Of course, the first big surge in female PhDs was in the mid-1990s, to which none of the preceding apply.  Given how many confounding variables there are here, ultimately, it's hard to know what explains this trend.