It is hard to say why those plants died without knowing more about the conditions and symptoms. However that said I suspect that soil moisture may be a factor. Soggy wet conditions (poor drainage) and dry conditions are both deadly to azaleas. Try to keep the soil evenly moist. Mulch the soil surface well to retain moisture and deter weeds. Fertilize monthly in spring to late summer with a product for acid loving plants. The plants like bright shade to dappled sun. If they were getting too much direct sun this could have added to their stresses.
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