I was just about to see if there was a thread on this after a friend at Williams posted this on Facebook. I’m happy that more students are finding out about such a wonderful college and taking the step to apply there. I’m especially happy about the increased diversity in the pool, which means a broader group of applicants is learning of the college and its superb offerings. I think one thing not addressed in the article is the new Coalition application Williams is offering along with many of its peers, which may have contributed to greater outreach. But ED applicants went up a similar percent as well (https://communications.williams.edu/news-releases/12_12_2016_earlydecision/) so it can’t just be that.
I did a quick calculation, and assuming Williams takes the same number of applicants this year, the acceptance rate will be ~1200/8577 or a 13.9% admit rate, which would be, I believe, the lowest in William’s history. As you pointed out, they took 11 more ED admits this year, so the RD/overall rate will be more competitive. Not the best news for applicants, but good news for the institution.