Prior to Euro-American settlement, fires historically burned through Ponderosa pine stands every 10 to 30 years. These fires tended to clear out grass, brush and small trees, leaving clumps of mature trees interspersed with meadows. Since the arrival of Euro-American settlers in the 1850s, the character of these Ponderosa pine forests have changed, with widespread tree-cutting for mining and homebuilding, livestock grazing, fire-suppression practices, and land development. Instead of frequent low-intensity surface fires, for much of the 20th century, these forests saw a distinct lack of fire.
As a result, Ponderosa pine forests have become more dense across very large areas of the Front Range, filling in those small meadows to create near uniform tree cover. When the regional climate turns warm and dry, like it did in Colorado this winter and spring, it sets the stage for a fire to easily start and spread.
This is a fairly informative article about the Ponderosa Pine. The article contrasts the ponderosa pine with the Lodgepole pine which depends on being burned and burns much differently than the ecologically "correct" way that Ponderosa Pines once burned. It also discusses the beetle-kill that affects both types of trees.