Photos would have been very useful, but if your trees have been in the same pots for eight years, I'd imagine they're desperate to be in larger pots, or in the ground. I don't know what size pots they're in currently, but you can check whether they're rootbound by turning them out of their pots and having a look. If the rootballs are solid, with roots wrapped round, then they need bigger containers.
Water shortage might be an issue, especially if they're rootbound. Potted plants in large containers in many parts of the UK during March and April do not often need watering, but this year, they certainly would have done because of the unusual warmth, and on a regular basis, as if it were summer.
The other option is frost damage - in the UK, this year, we had a summer like late March and early April, followed by a plunge in temperature of about 10 to 12 degrees and frost at night. The frost caught many new, tender leaves on perfectly hardy plants, but because the leaves were brand new, frost damage occurred.
Check all stems and backs of leaves for signs of insect infestation such as scale and particularly aphids, and for signs of disease - otherwise, photographs are necessary for any further diagnosis.