I think nomadism/pastoralism living around fragile ecosystem that forced people to just wake up - and move.
Normally what would happen - somone people would go prospecting for new land - pasture.
They would come and pick those who want to move - and they would move
Some would move far and get cut off - and become a new tribe.
Maasai who practiced pure nomadism - were forever moving - in circular way.
Kalenjin were agro-pastoral - so they need to get land that was dual purpose - with pasture and also good for agricluture.
Obviously through language and dna - it's now possible to know when they separated.
For Bantus - they were settled in fertile land and tended to their crops and few livestocks- and rarely needed to migrate - unless from over population and maybe attacks.
https://en.m.wikipedia.org/wiki/Baringo_County
I read them there the Torois. Appears Nilotes split so much especially on the river and lakes. The wars happening there do not have an end due to this. If in one location you find like 3 kabilas or subtribes then its quite interesting. Yoi would expect this kinds of splits on mountainous communities as they would be cut from rest of world but not on plains this is alot. Rivers and lake serve as transportation communication networks. Plains make it easy for travel.