Davis begins by making an extended point that the world is increasingly urbanized. It's not just more cities and bigger cities, but cities merging into one another making even larger urban areas, and rural areas increasingly urbanizing too, blurring the lines between rural and urban. Davis is not suggesting rural areas turn into metropolises, but that each geographic unit, no matter how small, is becoming larger and more urban than it was.
Davis links some urbanization in the Global South to the migration of manufacturing jobs to those countries (and specifically, to cities there). However, this is often not the case as many cities are growing even without manufacturing sectors. Furthermore, the size of a city's economy is not linked to the size of its population. Davis posits that the reason why cities grow even when their economies don't is due to neoliberal policies. Specifically, SAPs forced agricultural producers to produce commodities for the global market and those who couldn't compete were forced off the land. Former peasants move to cities, whether or not there is a job there for them.
The victims of this "agrarian crisis" flooded cities, even though cities did not have the capacity to adequately house them, educate them, or keep them healthy. Many who move to the city swell the size of the city's slums.
A review of Planet of Slums may be found here.