Thank you for your question, Steven.  The yew species has several pests, both diseases and insects.  Without some photos and/or a lab analysis, we can't diagnose which issue(s) they have.  Perhaps you can figure it out after reading these, first on diseases:  https://pnwhandbooks.org/plantdisease/host-disease/yew-taxus-spp-needle-blight

and https://pnwhandbooks.org/plantdisease/host-disease/cedar-port-orford-chamaecyparis-lawsoniana-root-rot

The common theme in both of these is water, especially non-draining water.  While that is often associated with available sunlight, the content of the soil and slope are important.  I don't know anything about your soil situation.

And here is one on a common insect pest:  https://hortsense.cahnrs.wsu.edu/fact-sheet/yew-root-weevils/

You'll need to do some sleuthing to see if you find insects.  If you can sent photos of the trees--up close to see the needles, as well as the whole tree--we can direct you further.  Good luck!

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Welcome. Sorry I couldn't diagnose the problem(s).  Perhaps you can take photos and plant samples to the Clackamas County Extension office for Master Gardeners to assess:  https://extension.oregonstate.edu/clackamas