aussie dendrobium dropping leaves :(

[neophyte]

my dendrobium hybrid is dropping leaves! i got it a couple years ago and it's bloomed twice now. it was a very robust plant, and i really don't want to lose it now, but I'm not sure why it's dropping its leaves. we had a few hot spells recently; not sure if that's the reason... the plant's potted in sphagnum, which I know isn't ideal for these plants, but I don't water very frequently, so I don't think it's root rot... your thoughts?

edit: a couple of the pseudobulbs look dark brown/black; they're actually just dark red due to sun exposure. the taller growth which lost its leaves is a new growth (grew this year). it lost its leaves before they even matured. the one with yellowing leaves is from last year, i think. both have yellowing pseudobulbs (or canes), and a couple other of the pseudobulbs are starting to turn yellow as well (they used to be green on the shaded side, red on the side exposed to the sun). not sure why this is happening...

second edit: just remembered to say that the plant was placed in the garage for a couple weeks while the new growth was maturing and got transferred back into the house a couple weeks ago. the leaves started to grow after the orchid was placed back into the house (the cane was still growing in the garage). so i'm not sure if the time spent in the garage has damaged it. hopefully this is just environmental damage and not a disease...