I wonder how much of them shift had to do with weather patterns and rain/drought.

Or it even could have been to let fields recover, while foraging was good/easy.

Both of those are definite contributors - even ancient peoples understood that leaving land 'fallow' for a year or two led to better yields than plowing and planting every year.

It's speculated by some, even, that climate change drove the entire 'migration period' which led Germanic tribes to overrun the Late Roman Empire, as their own lands became colder and less hospitable.