First of all keep the spike the to the Dendrobium that has the flowers wilting. You never know, it could grow a kheiki from there.
Second, this is a guess, but I feel it's an educated one without knowing what the Dendrobiums are...they're both hybrids. The parentage of your Dendrobiums may have Dendrobiums from the section Spathulata (the plant sizes can get very large in Dendrobiums within this section and are commonly known as "Antler" Dendrobiums collectively). Dendrobiums within this section are normally evergreen or semi-deciduous. So far that I know of Dendrobiums in section Spathulata are not fully deciduous.
Dendrobiums will drop old leaves though, so it shouldn't be a large concern unless the yellowing of the leaves are progressing up the cane pretty fast.
Check to see if the roots are damaged. If the medium needs to be changed out, I would do it immediately. Dendrobiums like to be very pot bound. An example of what I'm talking about is, I grow my Dendrobium hancockii which is around a foot tall in a 2 inch plastic pot (because the root mass fits snugly into a 2 inch pot). Because of this, I put the pot I have the plant in inside another pot.
Grow intermediate to warm with bright shade or bright indirect light.
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