But yes, you're onto something with the neighbour hypothesis too -- egalitarian and hierarchical (or beaurocratic and violent) societies often come in pairs, right next to each other.
The people retelling the story don't understand it - we have stories and kings and feudalism, go on the street and ask a random person any question about how feudalism works:
What rights did peasants have? What right does the lord have over his peasants? could peasants be sold and how are they different from slaves? Which 'levels' of nobility does what? Who pays taxes to whom?
If put theae people into a feudal society tomorrow, and just renamed 'baron' and 'lord' to something else, they would not know it. We could call it 'landlord' and 'investor'
This is going to sound sarcastic or something, but I'm actually curious -- do they? I would tend to assume that they appear to come in pairs, but this is largely because we happened to find history books written by the self-professed egalitarian society, and not their rivals, to whom they allocate every vice.
Their egalitarian neighbours where only just starting to discover. In part because their traces are less obvious, and in part because we've had a mental confirmation bias against finding them. I refer back to The Dawn of Everything if you want more details!
This seems like a pretty reasonable theory, but does indicate a little problem I think -- the people have experienced the feudal system will only be around for a couple generations after the change-over. So, at least to the extent that this is the cause, it would likely be a transitory configuration, right? (ignoring some unusual circumstances, like being neighbors to a particularly unpleasant feudal state and having them as a constant contrast). I wonder what that sort of configuration decays toward.