I just interviewed on campus this past Friday, but from what my interviewer told me the process works like this:
After you interview, your interviewer types the assessment/report
and submits it. For those doing off campus alumni interviews, this
probably takes a little longer. I was fortunate enough to interview
with a member of the admissions committee, so I anticipate that my
interview evaluation form will be submitted more quickly/efficiently.
After the form is submitted, it joins the rest of your application and
is moved into a pool of "complete" applications. At this point, your
application will be selected at random. So if my app. went complete
right after I interviewed, it could be reviewed now or 5 weeks from
now...luck of the draw. Once your application is selected, it is
reviewed by the committee and your admissions decision is rendered.
Logically, it seems that there will be some apps the admissions
committee will be unsure of, so it makes sense that most "waitlist"
decisions would come out at the deadline.