What is wrong with the pieris
The type of dieback you have leads us to believe that this is a root/vascular problem. Pieris japonica has a shallow root system.
It certainly could be excess rain-related- either that the roots could not get air to survive if that is a low or wet area, or because they can be susceptible to root rots.
They can also have borers or voles chewing bark or roots, etc.
We would investigate the base of the plant, especially where the yellowing branch with dieback connects, looking for any signs of gnawing, drilling, sawdust/frass, or other damage. On a dry day, you could go ahead and prune out that branch or any others with a lot of dieback.
These plants can be pruned back pretty hard and if the roots are healthy, will resprout with new branches and foliage next year.