Dendrobium Kingianum - Purple to yellowing leaves

[ebrookmi]

I have had this Den. Kingianum for ~1 year. Over the year, it grew two new growths and many roots, it had seemed to do well in my grow room. Seemingly overnight, the leaves developed purple mottling that has now turned into yellowing and dropping with the exception of a newly matured growth and the newest growth that is still developing. Many of the leafless canes existed that way at purchase. Here is the culture that the plant has received over the year:

Humidity between 70%-85% average
Day temps between 75-85F average
Night Temps between 68F and 72F
Watered daily (mounted)
Strong air movement
Fertilized using Ray's K-lite + Ca and Mg per container instructions for once per week feeding
Strong light (with a few of my cattleyas)

I'm wondering if this is typical winter transition behavior for Den. Kingianum (leaf drop), or maybe something else (or myself) is at fault? Any suggestions?