Are there any signs of holes or sawdust on the backs of the leaves? If so, it might be leaf or stem borers. Otherwise, I'd guess a water related problem (or a root rot caused by too much water in the planting hole that would get trapped by the surrounding clay) but I think it's a bit of a reach since your new growth didn't look effected yet.

Also, maybe the folks over in the Azalea and Rhododendron forum may have other options.