There are several reasons why prune tree leaves might curl, including:
Nutrient deficiencies: When a plant doesn't have enough nutrients, like nitrogen, phosphorus, potassium, manganese, boron, or zinc, the leaves may curl downward to conserve energy.
Pests: Tiny insects like aphids, mites, and caterpillars can feed on tree leaves and cause them to curl. Aphids can also produce honeydew, which attracts ants and can lead to sooty mold.

https://pnwhandbooks.org/insect/tree-fruit/plum-prune/plum-prune-aphid

Drought: If a tree isn't getting enough water, its leaves may curl or wilt. Young trees may need about a gallon of water every 7–10 days, but may need double that amount during hot, dry weather.

Fungal diseases: Leaf curl can also be a fungus disease that causes leaves to become twisted and distorted. In severe cases, the tree may defoliate.