There are many reasons a dendrobium orchid develops yellow leaves, and not all reasons are cause for concern. Knowing the type of dendrobium you have is essential to diagnosing the reason for the yellow leaves.
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.
A den-phal orchid, which is a dendrobium crossed with a phalaenopsis.
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.
A dendrobium orchid
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.