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On one hand it is natural for most conifers to shed their needles, however, the Douglas fir does not appear to be one of them.  You have reason to be concerned.  

Drought stress is a possibility, but there are two diseases associated with needle cast and several insects that can cause the same problem. I can't diagnose your tree without examining the needles but here's what to look for:
Rhabdocline Needlecast
Current years needles develop small yellow spots in late fall or early winter. Yellow spots enlarge and needles appear mottled. By spring the needle spots are a yellowish brown to reddish brown. Sometimes spots coalesce to discolor the entire needle. The discolored needles are cast in the spring after spore discharge.

Swiss Needlecast
Foliage symptoms include yellowing and browning of needles in late fall of the first year or during spring and summer of second year. Symptoms are most severe on older needles on lower branches. Diseased needles are often uniformly brown at the tips, but never mottled. infected needles are usually cast during late fall and winter.

Although fungicide applications can control the disease, a tree that is 50 per cent involved my not recover.
For positive diagnosis, take samples of the needles (alive and dead) to your local cooperative extension office.

See the attached website for more information:

https://www.canr.msu.edu/news/needle_shedding_in_conifers

After examination of your tree you might be able to correct some insect/disease issues, however, if your tree has been stressed by drought, you could have a problem that might need professional help.

See below for additional information:

https://extension.oregonstate.edu/news/conifers-naturally-shed-needles-summer-fall

https://extension.oregonstate.edu/forests/health-managment/diagnosis-table-douglas-fir-oak-pine

Good luck to you and your tree.