I planted about 10 arborvitae‘s probably five or six years ago. Some of them are nearly 8 feet tall. In the last month and a half, three of them appear to have died. Why so suddenly?
The arborvitae are evergreen trees that are known to have a fungus disease that needs to be treated. All the treatments has an active ingredient of Copper Sulfate. You can just buy the copper sulfate to get more for less than you would spend to treat the trees. There might not be any help for the trees that have died, but you can save the living trees by pruning and get some light to the base of the tree and spray to copper sulfate mixed with water around the base of the tree. Check in the base of the tree for a water puddle or a hole filled with dirt from decade foliage, and clean these hole out with a water hose and treat them with copper sulfate so fungus don't grow there. With any luck you can save the tree and it can return to a good health. They grow well in soil with low ph levels, acidic to alkaline.
This plant is notorious for suddenly dying. Where I live, we get very little rain in summer, so they have to be in irrigation their entire lives. It has very shallow roots. I worked in a nursery selling these for years. I hated them for this reason. I have talked to professional growers, the people that grow them for the garden centre. They have told me in summer they can never give them enough water. Another name for this plant is Swamp Cedar. If you Goggle Swamp Cedar you will get this plant. It's botanical name is Thuja occidentalis. I have included a link about this plant on the Missouri Botanical website, which has a great database for plants. It mentions being intolerant of dry soil. It also states, 'No serious insect or disease problems.' So, they most likely did not die from a disease.
Missouri Botanical Thuja occidentalis aka American or Eastern arborvitae