Sometimes ... a building is surrounded by structures that must be preserved. In this case, the blasters proceed with a true implosion, demolishing the building so that it collapses straight down into its own footprint (the total area at the base of the building). This feat requires such skill that only a handful of demolition companies in the world will attempt it. [HowStuffWorks]
...but we are supposed to believe fires perfectly imploded WTC 7.
Are you kidding me?