Chlorosis is a yellowing of normally green leaves due to a lack of chlorophyll. Many factors, singly or in combination, contribute to chlorosis.
A lack of mineral nutrients in the soil or prolonged drought could be to blame for the sudden yellowing of the raspberry leaves. Viruses, fungi, and bacteria can also cause the leaves to turn yellow.
The two most likely mineral deficiencies are of iron and magnesium. The symptoms are yellowing leaves, especially between the veins, with the veins remaining green. Acidic soils reduce the bioavailability of minerals. Thus, the soil pH could also be the reason why raspberry leaves turn yellow.