From the link, sounds like it is due to a combo of increased efforts by admissions personnel, especially for international students, and dropping the requirement for SAT subject tests.
I’d say it was overdue. The acceptance rate at Williams had stagnated over the past several years and the school had not been seeing the uptick in applications, and subsequent drop in acceptance rate, that many of its peers had been experiencing. Seems like some renewed efforts and a reformulated strategy did the trick.