It could be due to a number of things. A sudden increase in heat may be stressing the plants. While only one is affected this could be because other factors are creating a different environment for that plant. Lack of water is stressful but excess soil moistures is even more damaging when combined with heat. Sometimes the planting hole in a clay soil can become an underground bathtub holding water for an extended period of time. One plant's planting hole may be different in this regard. Other possible causes include damage from a herbicide used nearby such as a weed and feed turf product, root rots which can affect one plant but perhaps not (yet?) the others.
There is little you can do about these things except to try and maintain even moisture but especially avoid overwatering. .
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