Verticillium wilt is a serious soil born fungal disease that can affect Japanese Maples. While it is not the only cause of dieback, (frost damage is the biggest issue), it is present in the Pacific Northwest. A common symptom of this disease is multiple branches dying back one by one over several seasons. It will ultimately cause the death of the Japanese Maple.
If you are witnessing these symptoms, please bring in a branch sample to the nursery. One of our horticulturists can examine it and help diagnose the problem. If you are suffering from Verticillium Wilt, do not plant another Japanese Maple in the area. Choose another type of plant instead. Careful cleanup and sanitation may help control the spread of the problem. Do not compost diseased material.