Why do some vanda leaves turn yellow and drop?

[Ben_in_North_FLA]

I have seen many "palm trees" in my days in South Florida. In the vanda world palm trees are large vanda plants with just a couple or three rows of leaves at top and a long bare trunk between leaves and roots.
This is mostly due to the plant regulating the amount of foliage that it can sustain with the amount of moisture that it gets.
A well grown and hydrated vanda should not loose leaves at bottom unless there is a disease involved.
High humidity and watering commensurate with local conditions (more air movement requires more watering).
Generally well watered vandas with good air movement will dry completely in just a couple of hours.