There are fungal spores in the air.
In a humid environment they thrive.
I think from memory that they feed on protein from fingerprints, dust mite material, skin particles etc.
They use the minerals in the glass as a nutrient?
Some old glasses are almost immune to fungal attack. 150 year old lenses may be quite clean except for a natural surface blooming.
In binoculars, purging the air with nitrogen or argon, I think, guards against the problem, but only so far.
Typically it appears as spider patterns on the glass.
Either the glass or coatings are susceptible.
Australian optics in WW2 were rendered useless in 6 weeks from new.
So they had to learn how to clean them in the field.
Vast amounts of silica gel, I think, was used to store them.
I had a mothballed Williamson F52 camera completely encased in desiccant bags. This is a big camera with the 36inch Dallmeyer f/6.3 telephoto lens.
Tropics are the most demanding environment.
Some lenses, such as old Olympus zooms especially, often are useless from fungus.
So, a waterproof binocular is the way to go.
In the U.K, the temperature should be above 13C and humidity less than 50%.
Storing any optics in a typical garage in the U.K. is the same as throwing it in the garbage.
Storing in an attic is not much better.
Yes, being in a case can be worse.
I got a brand new Chinese waterproof binocular that already had fungus inside. So much for their makers environment.
This is a large topic.
P.S.
Fungus is rather easily cleaned off an external surface if seen early enough.
However, the fungus can track around the edge of the lens element into the interior of the optics.