Okay, I have seen this happen to me 3 times now maybe.
I am unsure if this is a new feature or a bug. I checked patch notes and also did "Find" for a certain text phrase and found nothing.
Dizzying swing is seeming to knock me back when I am blocking the attack with a shield.
I have 75% stamina and a Skeleton NPC went to do a Dizzying Swing attack on me I blocked at 75% stamina and it knocked me back.
In pvp this one person knocked me back twice at 50%+ stamina when I was blocking it. and it happened one other time.
I sure hope this is a bug and not a change.