Tree death mystery

We bought two pots of Box topiary bushes. Over the last couple of months they have both died. The leaves gradually turned yellow and dried out. I know that there is a disease called Box blight, but it didn’t look like that as it developed.
What is worse is that we had a three year old tree that was doing really well, but I have just discovered that it has died too. It budded and looked well, then all of a sudden developed the same symptoms as the Box pots that were within 6 feet of it.

 

First two pictures don't look like box to me.

 

They were box bushes trimmed into a pyramid shape. The picture below shows one before it died. The other photos are of the tree that died.

 

Well, cant think of anything that would kill different trees/ bushes at the same time.
Would weed killer spray drift be a possibility.

 

We have wondered about that as our neighbour lives often uses weedkiller. But the other plants around these three are all fine.

 

@IrisAtwood Just wondering what your weather has been like since Christmas and where you are based.
Is that new growth and healthy buds on the two trees and are they a Prunus species?
Most of the bark you can see on those two looks green and healthy. One part looks shrivelled, any twigs etc you think looks unhealthy just scrape some of the bark back if green underneath there is hope for that bit.

 

We’re in central UK. The weather has been very dry. Going to check the bark as you suggested.