a player being given damage from an entity causes knockback from the direction the damage was coming from:

summon an entity. then run

/damage @p 3 minecraft:arrow by @e[sort=nearest,limit=1,type=!player] 

The player will be knocked back in the opposite direction.
If the location is signaled instead, the player will be randomly thrown in some direction.

/damage @s 1 minecraft:arrow at ~ ~ ~1

This behaviour hasn't really been defined, but I think it would make sense for the 'at' signifier to declare the location the damage was dealt from.