I don't know how suitable this is, but I instantly thought of it as sort of comparable to bacteria in the wild, compared to the same bacteria moved to a sterile environment and being fed growth medium. The latter can grow to vastly larger quantities in a comparable area, maybe even in a giant vat. But if there's enough of a problem with the single source of growth medium, some kind of contamination or just no more supply, the whole colony dies. It's a more successful colony, but in a potentially far less stable state unless the conditions can continue to be kept that good.

The loss of roaming territory to the sedentary oppressors might also have something to do with the transition.

Also the foraging people might end up living on the periphery of a settlement, foraging and then trading what they foraged with the settlement to make thier lives easier.