tl/dr: Current and recent past regulations and means of forest management are responsible for the current wave of superfires.

Fires are historically common, and caused little major damage until modern management of land forests and forestfire fighting challenged nature and lost.

From the New York Times:

'Using data from tree ring studies, scientists have reconstructed a history of fires in the Southwest. The wildfires of the past were frequent and massive, but they stayed close to the ground and mainly helped prevent overcrowding. Take 1748. “Every mountain range we studied in the region was burning that year,” Dr. Allen said. “But those were surface fires, not destroying the forest but just keeping an open setting.” Cyclical wildfires were the norm.

But beginning in 1900, when railroads enabled the spread of livestock, cattle devoured the grassy surface fuels and the fire cycle stopped. A decade later, a national policy of forest fire suppression formalized this new normal. Over the next century, forest density went from 80 trees per acre to more than 1,000.

Then in 1996, the climate emerged from a wet cycle into a dry one — part of a natural cycle for this region. Winters became drier. And “we immediately began seeing major fires,” Dr. Allen said.'

http://green.blogs.nytimes.com/2012/06/25/goodbye-to-mountain-forests/?smid=tw-share