It's losing its leaves in response to the temperatures. Hong Kong Orchids are deciduous here. Are you protecting it from the frosts? They're pretty frost sensitive and given it's age and short time in the ground, your tree is more likely to die when a big freeze hits.

When did you plant it?

It can be from transplant stress, poor drainage or the cold weather.

Mine went through a transplant stress but quickly bounced back, and also had a case of chlorisis. Just sprayed the foliage with iron chelate and added some on the soil as well and it fixed the problem.

and as AJBB above said, Orhid trees don't like freezing temps. How cold has it gotten in your area in the past week? Mine's 10ft tall and was planted last spring. It's gotten down to 32-33f here this past week and the tree still looks good. I didn't cover it at all, but I do have two 90watt flood lights shining at it and a string of lights wrapped around the trunk. But its leaves are starting to get less green. This is normal during the winter.

Hi guys!

Yes, here in downtown Phoenix we had those 2 days last week that got down to freezing(but certainly not a hard frost...maybe in the 32-33 range at dawn is my guess), but I was able to cover the entire tree since it's still fairly short, and I put the C9 christmas lights(or C7, I always get them mixed up) around the base, underneath the sheets. It doesn't show any signs of frost damage, fortunately. So, yes, either it's bad drainage, chlorosis, or it's just normal stress and time for it to lose its leaves for the winter! I'm hoping for the last on that list!
True, AJBB, I probably should've planted it earlier (or later) to give it a better chance, but the cold weather really seemed to sneak up on us this year! Hopefully he will survive. :)

I'm assuming that once it does drop its leaves, I shouldn't bother watering it, but should probably still cover it if it gets close to freezing, no?

Thanks for your quick responses, AJBB and raimeiken!