IMO it's the weird aftercast animation (if you don't move at all, the character pulls the sword from the ground), that, even though cancelled by the movement input, roots you in place as if it is played out.
So far, what I've experienced, as I'm casting it myself and trying to make lots of different inputs, if you move immediately after casting it or you cast it while you're moving, the aftercast animation gets completely skipped, but if you stay in place for just a while too long and let your character start the animation of pulling the sword from the ground, even though a movement input will either speed the animation up or cancel it outright, the character still gets rooted in place as if the animation was being played out normally)