interesting piece of social rule

http://economix.blogs.nytimes.com/2010/04/20/a-tale-of-many-cities/

This has explanation of Zipf law according to which "the population of town is inversely proportional to its rank among cities. So the largest city has twice the population of second ranked,thrice the population of third ranked and so on."

Very interesting piece. Such elegant piece of rule in such a complicated real world scenario. Is this random or God playing its hand? Or there is a sincere explanation for this.

The network effect of aggregation of population can be understood but will this lead to such a clean relationship, that is difficult to explain.