RSO was originally turned on in the save but was removed before the affected chunks were generated.
Basically, when the biters are attacked at long range with artillery they run to the wall and most of them die to the turrets, however there are random amounts that end up sitting there.They are targeting the laser turrets (see below) but sit ever so slightly out of range. I feel like they are just getting stuck on other biters and then giving up on the pathing.

Save file: https://drive.google.com/file/d/1nig8RF ... sp=sharing
Check the map for the "Biters sitting around" markers they are around various points of the main base and outposts outside the walls.