The great Mwanamutapa Empire, ruled by Mwanamutapa, was founded in Zimbabwe in 1420. Industry provided the wealth of the empire. It became part of the gold trade routes that extended to China. Its primary industries were various metals: gold, iron, copper. Around 1490, the empire split and the Mwanamutapa took the north while the Changamire took the south. This may be what you are thinking of that reduced population in the 1400s as the kingdom thrived before the split.
The discovery of precious metals and natural resources such as copper and iron, combined with the centrality of trade routes, are factors that contribute to the growth of any major city. Certainly these were factors that we can see were key in the development of these locations.
Google Books is a great place to begin looking for information on highly specialized topics such as this one. (In fact, it's a good place to begin looking for information about practically any topic.) Google Books has a great advantage over Google the main search engine. The latter will return information about web sites prepared by anyone; Google Books will return information about published books. Often published books are peer-reviewed, and that is crucial: you want to make sure that you are using sources that have been read and assessed by experts. Here is a link that may prove helpful to you: http://www.google.com/search?q=african+life+before+the+slave+trade&btnG=Search+Books&tbm=bks&tbo=1#sclient=psy-ab&hl=en&tbo=1&tbm=bks&source=hp&q=Luba%2C+Kongo%2C+and+Zimbabwe+1400s&pbx=1&oq=Luba%2C+Kongo%2C+and+Zimbabwe+1400s&aq=f&aqi=&aql=1&gs_sm=e&gs_upl=304065l309538l4l309892l21l7l13l0l3l0l330l1364l22.214.171.124l12l0&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=33032504a0c2b7f5&biw=1366&bih=643
These states were all developed in part because of the natural advantages of their areas and in part because of trade. All of these states arose in areas that were good for agriculture but which also had things like deposits of copper and iron. These allowed the kingdoms to so some manufacturing as well. Being able to make things from metal led to the ability to trade with other peoples in the region. All of these things helped these areas develop.