Sudden death of azaleas and rhododendrons is almost always caused by Phytophthora Root Rot which is caused by poor drainage. The normal prevention is a raised bed with good soil and good drainage. There is no cure. Here are the possibilities:

1) Root strangulation. This is best prevented by proper root pruning when planting. If the plant is not too far gone, it might be rescued by digging and removing the soil. Then cutting any circling roots that may be strangling other roots. The roots need to be opened up. On larger plants, some of the top must be removed to compensate for the weak state of the roots. Any time the roots are exposed, they must be kept moistened. Roots that dry out will die. This could very much be your problem and pulling up the dead plant and looking at the roots will allow you to find out.

2) Phytophthora Root Rot or wilt. This root rot is the major killer of rhododendrons and azaleas. It develops when roots are growing in wet conditions. The rot is more prevalent in warm summer conditions. Plants infected with crown rot caused by the fungi Phytophthora have roots which become clogged with brown fungi internally. The roots get blocked and the plant wilts and dies. There is not much of any cure for crown rot. Some varieties of rhododendrons are vulnerable (Chionoides, Catawbiense Album, Nova Zembla) and some are resistant (Roseum Elegans, Scintillation, PJM). Sphagnum moss and bark dust combined with good drainage seem to prevent crown rot, but do not cure it. This could very well be your problem.

3) Drought. It may have simply dried out. If you plant rhododendrons or azaleas in late spring, it is very important to give them some extra water while they are growing new roots. Never let the soil completely dry out' it's best to keep the soil evenly moist. Too much water or poorly drained soil might be another explanation of sudden rhododendron or azalea death. Rhododendrons and azaleas have very fine, fibrous roots that are easily damaged by waterlogging, even for short periods of time. This is probably not your problem since it normally affects one branch at a time and not the entire plant.

4) Voles. Voles, also known as meadow mice, may have chewed on the bark and roots near the crown of the plant. Sometimes they chew all the way around the trunk and kill the inner bark, resulting in death of the whole plant. Keep mulch away from the trunk to discourage voles. Looking at the trunk the dead plant will allow you to see if this is your problem.

5) Bark Split. The bark may also split when there are wide fluctuations in temperature in the winter. Rhododendrons and azaleas may begin to come out of dormancy if late winter weather is warm; if a cold snap follows, bark injury is likely, especially in sunny, exposed sites. This normally occurs in the winter becomes apparent in the spring.

6) Borers. Rhododendron Stem Borer, Oberea myops, or Rhododendron Borer, Synanthedon rhododendri, is in a branch, the entire portions of a plant beyond (away from the roots) will wilt and die. Borers only affect the portion of the plant away from the roots from the borer. If the borer is in the main trunk, then the entire plant will wilt and die. The plant can be save by cutting off the area with the borer and letting the plant regenerate from the roots. There are no conventional insecticides that will kill stem borer larvae once they are inside the branches. The best control option for homeowners with only a few plants is to prune out and destroy wilting branches in early spring or late summer. This is probably not your problem since it normally affects one branch at a time and not the entire plant.

I have the same problem with mine and know that watering is not the problem. In some cases, the whole plant turns brown over a week or so OR various branches will die on a single plant and usually this death is first noticed by wilting leaves at the end of these branches.
I sent a diseased plant to Texas A and M University for diagnosis and was told it was a soil borne fungus, Armallaria, which attacks older plants due to weakened immune system due to age.

Armallaria is a naturally occuring fungus and occurs most often in oak forested areas.

As far as I know,there is nothing to kill armallaria. In addition, when cutting out the dead sections, you have to sterilze with bleach your cutters each time a cut is made.

This year, I will replace my infested azaleas with newer, younger ones and see if they are able to survive. I have researched the internet and found very few plants which are immune to armallaria.

Good luck

Armillaria is an exception and is actually worse after a drought.

I still mourn the loss of "Pink Diamond" It bloomed beautifully one spring, several weeks later it died.