OrchidTraci! I was starting to miss you!
Congrats on the Renanthera! That's great! About the misbehaving Dendrobiums: they need a LOT more light and fertilizer than people usually give them. They do NOT make good indoor houseplants in a window or something. They need Cattleya intensity light. Lots of it.
Since it's winter and the sun is not as strong as in the summer, give them some direct sun early in the morning and late in the day. They'll lose leaves and canes when they're trying to throw energy into new shoots and the light intensity / fertilizer is too weak. Of course, they'll lose old canes as they age, just as other orchids will lose old pseudobulbs. But it shouldn't be happening all at once.
If they seem like they're on their way out anyways, unpot them and check out the roots. You won't hurt anything at this point. If they're in bad shape and the medium is soggy--well, you know the drill!!