If a school has self-initiated interview it means that you don't have to be invited (as in turn in your application and receive a formal notification from the AdComs that you made the "interview cut") before you go on campus and do the interview. I don't think there are that many Top 15 schools left that do this-the two I know that take it very seriously are Duke and Tuck.
My personal take on the matter is that if a school has self-initiated interview, that it's a big old sign from them that they expect you to partake of its munificence, so book the plane ticket and go. The only school (to date) that dinged me sans interview had the option of self-initiated interview, which I didn't do because a) I didn't want to spend the money and b) it conflicted with the busiest time of the year at work. I tried to play the "but I'm a do-gooder who was so busssssssy with do-goodery" card on my app* and they still didn't buy it. YMMV.
*If I remember correctly, they actually had a separate question about why I hadn't taken advantage of it.