Every photo I've seen yet of "plume" is mold.
It hasn't been proven to exist in testing yet. Sometimes common knowledge and conventional wisdom is wrong.
The only thing I would consider to be something akin to plume is the faint crystallization you can see on a wrapper with a magnifying glass. But that doesn't grow or spread. And it wouldn't really show up in most photos.
Even mostly properly stored cigars can get mold, or bacterial growth due to any number of contaminants that could be on the wrappers.
But you're right, it's probably most often because it ended up overly warm or humidified at some point.
I also have a hunch that lack of air exchange can be a contributing factor. It certainly is with problematic mold and undesirable fungi in fungiculture.