Without more specific information or, especially, a photo, it's impossible to help.

Here's a pic -- looks worse today....

Appears to be a typical response to insufficient moisture at the roots. Usual reason for this is not roughing up the root ball enough at planting. The peat based potting medium dries out and becomes very difficult to rewet. Dig it up and soak for 30 minutes or so if the root ball is dry, then replant. Water when needed with a slow trickle at the plant base for the rest of the summer and fall.

The issue is complicated by the fact that the previous rhododendron in this spot died after 5 years - the explanation above certainly would not have applied after such a long time in the ground. It may have been a totally unrelated problem. This particular spot may have unusually sharp drainage that leads to more rapid drying out. Can't really think of any other possibilities.

Good points Mainegrower. Your point made is probably close to what's transpired here. Then throw in that the plant in question appears to be in 'full sun' all day doesn't help matters, does it?

I agree with Akamainegrower completely. That, coupled with a lot of sun, as Birdsong said, is the problem.
Good ground preparation, loose with plenty of organic matter, would make the roots spread out more so it could absorb water better in addition to roughing up the rootball.
Mike