Could be from several things. They may have prematurely defoliated in response to drought? It could be an Armillaria (one of a couple of species of Honey Fungus) or they could have suffered after last winter. Even though hardy, many shrubs have suffered from bark splitting after freezing solid in January. If it is this, they may well shoot from their bases and recover quite quickly over the next year or two.

.....Could be a combination of drought and aphid attack?