Some types of dendrobium orchids are deciduous. The leaves will yellow as the plant pulls the nutrients out of them and then all the leaves on the cane fall off. While this seems traumatic, it is a natural process for these types of orchids in their cycle to rebloom.
If just one or two of the dendrobium leaves turns yellow and drops, it may just be the plant jettisoning the oldest leaves. This is a natural occurrence and is no reason for alarm.
Yellow leaves can also be a sign of great stress. If your roots have rotted from being too wet or dessicated from being too dry, the leaves will turn yellow. Fungal, bacterial, or viral attacks can turn leaves yellow. Sunburn will turn the leaves yellow in blotches.