The events of Cadence's wedding just didn't add up to Twilight. Despite everything, the entire event went off without a hitch, which then left many unanswered questions. This kept on nagging her until she started to put the pieces together. After convincing herself that there was simply no other way, she decides it's time she confronted Celestia and Luna about it.

What she learns is both shocking and educational. Shocking because of what she learned, and educational as to how far the Princesses were willing to go to ensure the safety of their land and its subjects.