Waitlist decisions are made as other students give up their spots. This will continue to accelerate as more schools, especially many of the T20 schools grant their acceptances. You can appreciate that there are top students holding upwards of 4-8 acceptances right now. Those same top notch students, if accepted to a T20 school, may then release those 4+ acceptances. Of course, after April 15th, students will be limited to the number of acceptances they can hold and by May 1, the WL positions will continue to open up.
One important thing to note is that many schools do not compose an ordered WL, though they might sort the WL students into tiers. ADCOMS are looking to seat a diverse, balanced class of students, so if an OOS URIM basic science research-y student gives up an acceptance, they will likely look to replace the spot with a similar WL student, and not necessarily the "next" student on the list. And some ADCOMS will look more favorably on a student they think will accept the offer, especially as the season winds down, as it will be time to concentrate on focusing on the next interview season come summer. However, as many have written on this site, it may be difficult to know which desired WL students would drop school X to attend their school, as many students are less than honest in the process. This is when things like knowing an applicant's current mentor, or applicant already located in same city, etc may come into play.
