That looks like classic wind and sun damage. Typically during cold weather when the leaves were curled up, wind and sun desiccated the exposed portion of the leaves when the ground was frozen or the roots were too dry. Rhododendrons need watering in winter to avoid this. I don't know where you are in Oregon, but this is unusual for a hardy plant in Oregon. If you are in the Willamette Valley, you normally have mild wet winters.

Large leaved rhododendrons are susceptible to sunscald if the plants do not receive enough moisture before the soil freezes. The leaves curl in cold winter weather, exposing the central part of the leaf but not the edges, resulting in the area near the main vein becomes desiccated and turns brown. To prevent this, water plants thoroughly before the first hard frost, protect from drying winds, apply mulch, and locate in partial shade.

Here is a link that might be useful: Wind and sun damage.