I've got 3 of these dwarf trees, about 5 years old, and all of them have started yellowing like this

elderberry

What could be the problem? They've been fed this year, by top dressing with rotted manure several months ago. The yellowing only occurred in the last month though

Dovefromabove

I would say that’s due to lack of water at some stage. 

stephen.mcmellen.49

The main problem is these should be in the ground,  but it not, a much larger pot will help considerably. 

Fairygirl

Really not ideal in pots @elderberry , unless you keep re potting, amending the soil regularly, and keeping them watered. 
The tree canopy prevents water getting in too, so even during rainier periods, the soil can be bone dry. Rain has to be steady and consistent for hours to get through. That hasn't happened recently - they've been affected for a while, but it can take a while for it to be noticeable.
Even in a wet area like mine, you have to be vigilant. Just mulching with manure won't really do any good, and once they dry out, it can be difficult to get them dampened again. You need to thoroughly hydrate them by either putting the pot into a bigger container of water until there are no bubbles appearing [that means the soil is properly dampened] or you can place a tray under them and water, and repeat until they're no longer soaking it up. Keep them in a shadier space and sort out the soil and hydration, and they might recover.
It's normal for the new growth to be on the outside, and for the inner parts to go brown, but that's more like a hydration problem