Beginner Gardening: Browning/dead Emerald Thuja trees, 1 by hoagies

hoagies wrote:
I planted several 9' Thuja trees for a friend during the last week of June, 2010. I chose all trees myself from a reputable nursery, and all were in excellent condition when purchased. July and August were very hot and dry in the Alexandria, Va. area. The trees get a southwest sun exposure. I'm told the trees were watered every 10 days or so. Within three months most of the trees have browned and are now dead. What would cause these once vibrant trees to die so quickly? I see no evidence of bugs. Any suggestions?