I do not know what the exact triggers are, but if you are block when a DWing enemy is doing their power attack, you can get locked. I do not know if you actually have to get hit for it to happen or not, but it does happen. Twice now for me, in fact.
Oh, and I also would like to point out that after you use your standing power attack while dual-wielding, armor on your character's shoulders will shift in an odd manner. Although there's a plethora of issues with dual-wielding as it stands, so meh.
