Pine needles live for different lengths of time and then turn brown and drop. These will be the innermost (oldest) needles. This could be what you are seeing.

See: http://byf.unl.edu/natural-needle-drop

The mountain pine beetle is native to western North America. Preventative sprays can protect valuable pine trees, but the mountain pine beetle only attacks pine trees. Particularly ponderosa, lodgepole, Scotch and limber pine. Bristlecone and pinyon pine are less commonly attacked. http://www.ext.colostate.edu/pubs/columngw/gr090406.html The Douglas-fir beetle (D. pseudotsugae), is a related insect and occasionally damages Douglas-fir. Most often, outbreaks are associated with previous injury by fire or western spruce budworm. Spruce beetle (D. rufipennis) is a pest of Engelmann and Colorado blue spruce in Colorado. Injured pines also can be attacked by the red turpentine beetle (D. valens). http://www.ext.colostate.edu/pubs/insect/05528.html For more information on spraying pine trees, see: http://www.ext.colostate.edu/pubs/ag_natr/spray-trees.pdf

Additionally, you can contact your local US Forest Service for more information.