Those stressors could have pushed even rainy cities past what the local societies could tolerate, as mutually reinforcing crises rippled into each other.

Like, think back to the "Arab spring" when mass migration attempts were made towards Western Europe (some of it continuing today). In geological history context, a thousand years from now, that's within the timeframe examined in the article for the past, given Europe is now experiencing more drought-like conditions itself as well.

Or think of South American mass-migration attempts from a few years ago, that went all the way up to Canada. Never mind the grab for Ukraine's most prolific wheat production areas (I'll leave it up to the reader to search up maps for this - they do differ by years for example but we can see a general tendency towards anything east of the Dnipro river being a really good area to have under your control in the future.