Originally Posted by Think Green
Mite damage, thrips, phomopsis blights, Drought, heat.....lack of ample soil moisture.............all threats to these arbor's.
Without lab identification, we are just whistling into the wind for answers. Don't rule out nematode damage either.
In this case it was Very Obvious Operator error. When different shrubs all have the same burnt fronts and Health back sides it is NOT Biological damage. All the pests you listed would have a more uniform response to the whole plant and not just the side facing out.