Comment by XorNot

This is exactly it. The other part is not just water pumping but operating the sewer systems - if the lift stations are down the whole thing fills up in about a week and the basic plumbing in your house - and thus pretty much entire city, stops working.

Cities are not setup to support their current populations without those services and once you run out of buffer things go downhill quick - wastewater is an enormous and immediate disease hazard.