Currently, using a shield to knock back an opponent doesn’t cancel the opponent’s shieldwall – contrary to the description of the skill. I’m sure it did get cancelled formerly. Possibly it’s just in specific circumstances that it doesn’t cancel …
UPDATE: just tried it again in the same battle, and this time the shieldwall did get cancelled. Both instances knocked the opponent down a level of height, as well. I’ll see if I can work out what the difference is …