When you've gained enough influence with Leliana, she will tell you the real reason as to why she left Orlais. Later on during a random encounter you will be ambushed by a group of bandits/assassins hired to take out Leliana. After the fight she interrogates their leader and finds out that you will find your answers in Denerim. Go to Denerim Market District and it should be at the nearest building you can enter to the right. There you will find Majorlaine. Kill her and tell Leliana to embrace the killer in her.