The smaller cane may actually be an older cane from when the plant was smaller or near blooming size. If that is the case, it is normal for older canes to drop their leaves. They will slowly dry out and you can remove it when it dries out if you'd like. I leave the older canes on until they dry out, because even without leaves they remain healthy and act like batteries that can store energy for the rest of the plant. If it is a new growth since you bought it, you may have an issue with roots or the sun levels may be too low. Dendrobiums are sun loving plants. They can take all but full sun if you slowly move them into higher sun levels. Best of luck, it is a gorgeous thing!