You are ranked at each step. Before you interview, your GPA+MCAT+the rest of your app --> puts you somewhere in a rolling stack of applications. Then each week or so, when the adcom meets, they take the top X amount and invite them for an interview.
Then, after you interview, the adcom meets and discusses the candidates who have been interviewed. They rank them, now based on the previous criteria PLUS their perception of the interview. If you're in the top X amount, you get accepted. If they don't think you'd be a good choice for their program, you get rejected. If they think you'd be a good fit but they'd like to wait and determine how you stack up against the other candidates whom they interview this year, they waitlist you.
Contrary to popular belief, being waitlisted does not necessarily mean you "bombed the interview." You are constantly being ranked and re-ranked, and you happened to fall into a spot that was acceptable to them, but not stellar. The waitlist is constantly shifting, too, though. So you could be low on it now and work your way up. Or you could be high and work your way down.
The best thing to do is to try and ask them if there is something you can improve on to be a better candidate. If they won't tell you specifics, then just look back and see if anything stands out in your mind. Were your interview skills weak? Did you answer a question in a way you don't think was optimal? Were you uneducated about that specific program?
Think back and try to correct any mistakes. But if you can't think of any, don't dwell. You'll only psyche yourself out for the next interview.
You have 5 interviews, and there is a good chance that you wouldn't be accepted by all 5. It's just how things often work out. Just try to improve for your next ones, and don't worry about the waitlist. It's later in the season, and most schools have already filled a decent portion of their class. Thus, your chances of being waitlisted in Dec/Jan are greater than Aug/Sept/Oct.
Good luck!