When survivors get knocked back from a range attack, do they get pushed in the cardinal direction closest to the vector from monster to survivor?
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.