Answer: Unfortunately I don't have a magic explanation. Since you are an experienced gardener and started your own plants and so on I'll assume the plants were not stressed prior to planting and were not root bound prior to planting, were planted correctly and well cared for, so the next possible reason I can think of is that there is something going on with the soil in the location where you planted them. You might find through soil tests that the soil in that spot is out of kilter or you might find through some careful investigation that something once "happened" in that spot (a gasoline spill, there is an odd batch of subsoil there, someone used to burn trash in that spot, etc.); there is also a possibility that a tree root has encroached or there is a large rock below the surface or perhaps your irrigation system did not function fully there, or some other logical explanation. Finally, there is always a possibility of fertilizer burn or some other accidental cause during the growing season. I hope this will help you figure out what happened.
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