The rules say knockback occurs directly away from the monster, so if the vector from monster to survivor is more Southward than Eastward, is the knockback wholly Southward?
There's no official answer to this from the FAQ, or anywhere in the rest of the community, unfortunately. Most people play that you would do it in the rough direction of the vector, alternating moving horizontally and vertically in a way as to match the line between the survivor and monster.
This makes some of the monster abilities work more sensibly - there is a monster that does knockback towards itself, pulling the survivors to it (ostensibly to then beat them to a pulp). Doing the knockback in a cardinal-direction line would prevent this from working as intended, and potentially leave the survivors out of reach.
My understand is that the knockback direction is perpendicular to the monsters movement. That is to say if the monster is moving vertically, the survivor is knocked horizontally, and vice versa. This also means you will not have multiple collisions with the same monster/survivor except in very rare cases.