I have often said that our evolution and psychology is better-suited to the hunter-gatherer lifestyle of small, spatially-separated communities.

It did occur to me that, when some communities discovered that staying in the same place and actually cultivating plants and domesticating animals could actually produce not only a sufficiency, but an actual surplus, was when the problems arose.

It is far from improbable that some of the still-nomadic communities realised that they could use their hunting skills and tactics to take some if not all of the surplus from the settled communities in order to see themselves through the winter/dry season/monsoon (delete as appropriate). This would lead to settled communities merging and 'digging in', for better defence.

It is also possible that at some point, the leaders of a settled community approached the leaders of a nomadic one and offered to feed asnd house them in exchange for protection against other nomads. The more aggressive and better-armed nomads would ultimately evolve from mercenaries into rulers, and we get the beginnings of a feudal system.

The rest duly becomes history (after some bright spark invents writing.)