Because there is no such thing as time in the Dark Dimension, The Ancient One tapped into it in order to extend her own life.
The first blow comes at the end of the second act, when Doctor Strange makes the unfortunate decision to reveal a dark secret that The Ancient One has been keeping — potentially for centuries.
While the Sorcerer Supreme forbade any of her followers from abusing dark magic and tapping into the power of Dormammu in the Dark Dimension, it turns out that she was actually doing it herself.
It was discovering this that led Kaecilius (Mads Mikkelson) to see her as a hypocrite and seek Dormammu’s power for himself — but this was kept hidden from Mordo. When he does finally learn about it from Strange in the New York Sanctum Sanctorum, it most definitely rocks him to his core — as he truly put all of his confidence in the ways of The Ancient One, and was very clearly disturbed to learn that she isn’t the perfect force for good that she portrayed herself as.