Viburnum mystery death

Any ideas what caused five Viburnum suspensum to die?



Here's the symptoms close-up and from a distance. Plants are in hedge with just one affected area, other plants are fine.

• Monocots (cabbage palm seedlings and grasses) were not affected. Brazilian pepper seedlings were fried also.
• The cambium is dead on these 5 plants, so they aren't going to come back easily.
• These are on a slope & mulch is less than 2 inches.
• No indication of heavy fertilizer applications.
• No obvious oily residue on mulch or ground.
• The row of Japanese ligustrum in the same bed, but on the backside had some minor symptoms, no big deal.
• Oak tree isn't affected.

All ideas appreciated. I'm guessing we will never find out, but can't resist the diagnostic quest!