Michael Reilly provides a much more satisfying explanation at Discovery News. Much of the valley where Guatemala City is located is covered by up to 600 feet of loose pumice and volcanic ash, materials that can be easily eroded. When sewer pipes or storm drains break or leak, they can start eroding the loose materials deep below the surface, eventually causing the surface to collapse inwards. In the Discovery News article, geologist Sam Bonis refers to the feature as a piping feature. This isn't the first time such pits have caused problems in the city; in 2007, the pipe below destroyed twelve houses and killed three people (thanks to Discovery News for the link). The stench of sewage and the sounds of flowing water in the 330-foot-deep hole confirmed the origin of the problem.
UPDATE: Oops, geologist Sam Bonis called a piping 'feature', not a 'structure'. I've made corrections.