Why are leaves yellowing on my Dendrobium Biggibum?

[nidiasophia]

HI,

I have a Dendrobium Biggibum (or cooktown as they call it) orchid that is growing a new cane. Everything appeared to be ok, but I am noticing now that leaves on the new cane are turning yellow and falling off.

The problem is that it is not the leaves at the bottom, but leaves closer to the top that are turning yellow and falling off. See the picture for details.

Right now I have the orchid on a windowsill in my office that faces southwest and gets alot of sun.

Any advice?

Thanks