there were several seemingly destabilizing factors, sort of a perfect storm, each contributing to further disconnect and polarization.

the group grew too large (and displaced other groups), but then ended competing for the best food among themselves, and having trouble socializing and bonding in such a large group.

subgroups forming, first fluid but eventually creating a split

loss of older alpha males exacerbating competition between males

loss of the few individuals that still maintained some relationship with the other group (the last one doing so actually died in that epidemic while the split was already well underway)

it is indeed an amazing read. my take away is that the root cause was mainly the group becoming too large, this affected socialization and cohesion, and thus the group was unable to cope with everything that came after.

Edit : I just read the paper and the discussion does a good job at laying out the entire landscape that contributed to the disruption. Pretty fascinating but also totally explainable due to the circumstances explained, which in and of itself is wildly fascinating!