Thuja plicata - browning after mulching

I have three Western Red Cedars that have been in the ground around 15 months and seemed quite healthy. Around a month ago I thought it would be an idea to add some manure and bark chippings around the base of them, and now they're turning brown, from the inside out it seems.

Presumably this isn't a coincidence, so what could it be in the mulching that is causing this?

 

Don't think it's caused by the mulch, if it was the trees would be going brown all over not just the odd branch here and there.
I'd just cut the brown parts out and keep a close eye on them.

 

Well I removed the brown bits and there are no more brown bits since.

To be fair, if I were these trees and I wanted to get rid of some leaves that were kinda hidden away in the middle and not very efficient at catching the light, I would have chosen these ones. Maybe they were making themselves shipshape against next season. Is that a thing?

 

The inner leaves on conifers always die, that's why you cant cut back into brown wood, as it wont regrow very well, if at all.