It really is an interesting phenomenon. I sort of get cities changing tacks when they've been really successful. It's at least plausible that the same regulatory and policy approach that allowed your city to prosper might not be appropriate when you get to a larger size, and then there is the natural tendency of established parties to want to pull the ladder up after them so to speak, and protect their position and stop the city from continuing to change now that they have what they want.
But it's interesting to me that it seemingly goes hand in hand with adopting national democratic policies and electing particularly corrupt politicians, when just the average level of corruptness would easily be born by the city. Is it cities are more attractive to people with those tendencies? Does dense population make people more receptive to collectivist policies? Even if that's the case, why can't normally corrupt politicians prosper? You have the occasional Guiliana or Bloomberg, but the Deblasio, Wheeler, and Lightfoot seem to be par for the course. Just a weird phenomenon to me.