Why do some vanda leaves turn yellow and drop?

[Roberta]

I suspect that there's going to be variation in the cold tolerance of Vanda hybrids, depending on ancestry. Even if most of of them are predominantly V. sanderiana, the amount of other species is going to vary - and some are pickier than others. So the little temperature jolt will get laughed off by most, but some will be "offended". Getting down into the weeds of pedigree may help to identify which are the most sensitive ones.