Just in terms of the chemistry, crystals usually form in a solution that is supersaturated with a compound (e.g., salt in hot water or table sugar in hot water), as the solution cools, and is no longer capable of keeping the solute in solution. From tobacco leaf, I can easily imagine some dissolved compound on the surface of the leaf crystallizing as the leaf dries, or as the lamina shrink during kilning.
The contrary seems to be the case with cigars that are properly kept in a humidor. I can see no reason why anything would crystallize on the surface of a cigar kept a constant 65%+ humidity. Volatile compounds are indeed released, but they are volatile, and don't precipitate back onto the surface from which they were released. (The common practice is to maintain tobacconist humidors at a nominal 70%--conditions which offer a nice, "fresh" feel to the cigars, but can support vegetative mold growth.)
I agree with JBD about crystals on cased commercial tobacco. If the tobacco begins to dry, the glop in the casing can crystallize.
An easy method for telling the difference between crystals and mold is a high power hand lens (10x to 30x). If you see hyphae (branching filaments), it's fungal. Also, if you see hyphae, they surely penetrate into the lamina. [Search Google for images of hyphae.]
All that being said, for all the decades that I've been storing cigars at 65% RH, and the years that I've been kilning tobacco, I can't recall seeing crystals.
I have one curious exception. Decades ago, I had a box of cigars (La Herencia, if I recall) that were cello wrapped, and were contained in a Spanish Cedar box which had been made from inadequately cured wood. I didn't like the cigars, so I just ignored them in my humidor for several years. When I finally opened the box again, there were crystals (which I assumed were some terpene compound from the now dried wood) precipitated onto the exterior of the individual cellophane tubes. So crystals were there, but they were not from the cigars. And it's the only box that did that, among 45 years of cigar boxes.
Bob