I believe it's because the ability timer is tied to the buff (Major Sorcery/Brutality) rather than the skill itself. When something refreshes the buff it provides it clears the timer. Unfortunately this isn't the only skill with this issue. Degeneration, Swarm (and morphs), and Expansive Frost Cloak are examples of other skills that run into this issue as well. It's fun watching your ability timer on Swarm go to like 36 seconds when someone procs Nazaray.