It was indeed explained. At the start of fragment eight, Sullivan left a letter for Deirdre that we were able to read. The long story short - he had seen some amazing things from magiq, but he’d also seen people totally destroyed from magiq. He didn’t want her pining for the hidden truth of magiq, thus endangering her life, so he thought she would be better off as a regular person. To protect her, he created a spell called the Calling of the Corners, that he performed with six friends (each from one of the guilds).
Deep down, though, he really wanted Deirdre to know the truth of him and of magiq. When he was creating the spell, those feelings got mixed in. You can read about that in the Monarch Papers, Vol. 3. Because of that, the spell had a loophole that, if performed twice on the same person, would free them from the protection. Using that, we were able to perform the spell on Deirdre and free her. And, well, that’s how we got to where we are now - Deirdre was finally able to understand her dad’s journal and now she’s following it to find the truth.