Was there any kind of land disturbance in the area of the shrubs - construction, new hardscape, any kind of disturbance? Sounds like they are dying back. I have similar symptoms on a little holly that I had to move. It was fine for a while and then suddenly the leaves became brittle as you describe and the plant is dying, perhaps dead.
Even though there is plenty of water, something can be happening to the roots to prevent the plant from taking in water. Some ideas are animals tunneling beneath the plants like voles, something eating the roots like grubs, a competing nearby tree such as a maple taking up the water supply, bark damage preventing uptake of water or nutrients.
Look at the stems closely. Do they show some shriveling?
Any stems like this can be cut back. Do you see signs of any kind of pest? Do you see webbing? Do you see any markings on the leaves like white stipling above or below. Any scale attached to the wood? Also look for cankers, splits or bark damage on the stems.