(Just my guess) They break formation when their individual path finding takes over and they all think they are running into the one in front of them and try to find their way around. Which runs them into other biters, continuing the problems.
Or possibly the spitters stop when they reach their range, causing biters to have to path around them?
Yea, once the group encounters the wall, they all individually try to path to the wall. However they collide with eachother, so keep trying to path and repath and path and repath around the other biters who are also pathing and repathing.
As an experiment I made biters not collide with other biters, and this is the result:
Before: https://gfycat.com/ambitiouslighthuman
After: https://gfycat.com/deliriousfabulousbumblebee
Edit: I made them not collide in my recent combat mod, KS Combat, along with some other tweaks to try and make groups move nicer.
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