Why do some vanda leaves turn yellow and drop?

[Keysguy]

OK, so I'm putting this question here in the beginner's forum in spite of my possibly misguided self-esteem that I am no longer a beginner at this crazy hobby. I've done a search and although there are historical threads that touch around my question, none seemed to specifically address what I'm going to ask. So, here goes.

I have maybe somewhere in the neighborhood of 50 vandaceous orchids hanging in my shadehouse here in the Florida Keys.

I have an off and on situation where some of the plants will have bottom leaves turn yellow and eventually drop but it doesn't happen to all of them (maybe 10% +/-). I have several that have never lost a single one. I'm going through a spell right now where there are quite a few actually and the only thing I can think of is we had a little "cold snap" here while I was up north for the holidays. And by that I mean maybe it got to 55F for a couple of hours. I wouldn't think that would be cold enough to cause that kind of stress but what I'm noodling on is whether it was cold enough coupled with watering from the automated misters?

This isn't a catastrophic thing, I'm just trying to figure out "why" and then correct a culture problem if I have one.

Like I said, they don't all do it and I've never been able to put my finger on a cause before but those couple of chilly nights are making me wonder....... if that might be "a" cause or contributing factor. And that's not to say they don't do it other times of the year because they occasionally do.

Anyone have thoughts?