Banshees veil popped by drag?

So maybe I just barely notice it after playing a match as but when I had the spell shield popped off of its attacks on me. They didn't contest drag so it didn't make a difference but why does drags attack pop the passive shield? Is it the slow applied by dragon that pops it? Has it always happened and I never noticed? I even read the I mean it says blocks the next enemy ability not an epic monsters attack. I would get a baron knock up, but not the dragons normal attack