As a disclaimer, when I applied to Midwestern several years ago, I wasn't personally placed on the interview waitlist but I knew applicants who were. But from what I understand and what they relayed to me, while you theoretically can be called off anytime, it is most likely to happen more towards the end of the cycle. Unfortunately, some people just don't get called off at all; I believe that it's contingent upon there still being vacancies in the class after admissions has already gone through the accepted applicants and the post-interview waitlist first.
@FrostySparrow can probably confirm or correct me if I am wrong.