Many times when bushes and trees dies there can be be fungal diseases involved. Particularly when they are weakened from old age, too much water or other adverse conditions.
So I looked up Azaleas and it seems that Phytophthora root rot can be a very common problem with them. This often gets worse if the soil has been wet, especially in combination with high temperatures.
There apparently can be interveinal chlorosis and also a little purple in the foliage, depending on the cultivar. This sounds a lot like what has happened to your azalea.
You can do a search about it, or you can read more about it here:
http://www.ces.ncsu.edu/depts/...
A list of cultivars and their resistance to this disease:
http://www.ces.ncsu.edu/depts/...
I think Phytophthora root rot is a strong suspect here, but even if it's something else, I think that it's best to not put a similar plant in this place for a while. The earth will be tired after growing one species on it so long and other pathogens can build up over time as well. Perhaps also it can be good to amend the soil a bit and raise the bed some, especially if you feel that it gets a little to wet at times.
