It's not really a common thing. They do a second interview if they feel that they are unsure about you and have yet to make a decision on you. This is what they told my group. Pretty much, if they don't like your first interview but feel like you still have a chance? (I don't know, just my speculation) But if you have been rejected or waitlisted, I would not expect a second interview. I think they RARELY do it and its for people that they have not made a decision on. Good luck!