Congratulations on being asked to interview! That's a great accomplishment, never a bad thing. Were you expecting to be admitted without having to do an interview? Or did the invite come after round 2 offers. If it's the latter, I wouldn't overthink it too much. Often times, schools send out interview notices in a haphazard fashion. The good news is you got one, now what are you going to do with it? Make sure you are adequately prepared. Good luck!