MOST LIKELY it is due to your interview performance, or something you said in the interview. Although this can be a lot of things. It's entirely possible, maybe even likely, that you mostly interviewed well... but you answered a question in a way that sent up red flags. A big example of this is how you answer questions about red flags, previous grades, misdemeanor charges, and general attitude.
At some schools (LUCOM), you might actually be rejected post-interview for your grades or mcat. I know, right?
Also, keep in mind that your chances of being waitlisted/rejected post-interview increase dramatically when it gets to be later in the cycle.