Not certain how set you are on leylines being fixed, but it would be interesting if the Phaen's unpredictability was -- at least in part -- linked to roaming leylines. If you assume that proximity to a leyline augments your strength (I feel like this was implied), you'd be unprepared if the Phaen leyline wandered nearby while casting. The path of Phaen's leyline through the magical world could act like lightning, always following the path of least resistance through the air. Mastery of Phaen energy would then become more about predicting or controlling leyline movements.