This due to the bark beetle which is thriving due to the increasingly mild winters. It’s chomping through the lodgepole pines at an alarming rate, decimating entire forests, killing over 4.8 million trees last year.
A cold snap would kill many of the beetles, but so far that’s not happening.
This photo from a Canadian government site shows the extent of the problem there too. The red trees have been killed by the bark beetle.