Is my tree dying and why?

[Peasgood]

Recently noticed a lot of dead foliage on my trees and would like to know what is causing it. 

Young/new growth looks very healthy. There are three trees and it appears to be progressing with the least affected just yellowing and the worst tree very brown. Tried to show the difference in the two pics. 

[Peasgood]

At the moment it appears what is going to die has done so and the younger foliage is fine (no yellowing)

I've known the trees for decades and don't recall them doing this before. No damage, digging, waterlogging or anything springs to mind. They grow on the banks of a stream so wouldn't think drought

 

The top pic shows one that is yellowing and the other shows how they seem to end up, at least the rest isn't yellowing so I think they will survive. Change seems to have been in the last fortnight or so. 

Quite puzzled by this and will feel a right tit if they turn out to do it every year.

[The avantgardener]

It looks like Cypress aphid (Cinara cuppressi) damage. If so, they will overwinter as eggs and cause damage next year. Inspect the trees next year in May/June and see if you can spot them, quite large aphids with two dark stripes. If found, 

treat with an appropriate insecticide.

[Peasgood]

23 minutes ago, The avantgardener said:

It looks like Cypress aphid (Cinara cuppressi) damage. If so, they will overwinter as eggs and cause damage next year. Inspect the trees next year in May/June and see if you can spot them, quite large aphids with two dark stripes. If found, 

treat with an appropriate insecticide.

Thanks for that. I very much appreciate your post but I'm not convinced. Google search suggests young growth also suffers but on these trees young growth is fine, looks very healthy. Maybe it was aphids that have been and gone so will monitor for further changes.