When a fully charged sword does a sweeping attack, the area-of-effect knockback effect is being applied to invisible armor stands.

For the length of the invisible armor stand's existence, it has always never been manipulated by its environment, an important feature for map-makers.

The sweep attack undermines this 'tradition'

Steps to reproduce:
1) Run the following command: "/summon ArmorStand ~ ~ ~

"
2) Go into spectator mode and observe the armor stand's position.
3) Summon an attackable mob, and then attack it at full power with a sword while near the armor stand.
4) Observe the armor stand's final position. You should see that it moved.