Based upon what you have written... the most probable answer to your question in my analysis would be...
You've been Stoked. This means you are being called for a Second Marriage Interview because the examiner based on your interview and the evidence you submitted had some doubts about the "bona fides" of your marriage. There are many more questions that you can expect to be asked at this next interview. You can expect to be asked to appear at a different location, and to be separated. If you have further evidence of your marriage, it would be a good idea to bring that evidence, and to keep a copy. Perhaps your answers were inconsistent or said with a voice sounding equivocal. Sometimes the precision of the answers if too exact or too off can lead to suspicion compared to answers that are close but given from different perceptions. Such as: We have two tv's and one broken monitor in the back room, and we have 3 tv's.
I would strongly consider consulting/hiring a competent immigration lawyer to review your file with you, including what was said at your first interview, and to prepare for this interview.
The apparent demeanor of your adjudicator/administrative officer does not always correctly imply the decision they will be making on your file.
Hi. Usually a second interview means that the officer has doubts as to whether your marriage is real (bona fides).....second interviews usually mean that the couple will be split up....and asked similar questions in order to determine whether their answers match. I would strongly recommend that you to the second interview with an attorney. Wish you luck!