The specimen degraded
Updated
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Pyrite oxidation
A white or yellow bloom, sometimes a sulfurous smell, then cracking. It needs humidity, so a sealed container with a live desiccant in a cool stable place is the response, and it is the main reason to seal a fossil jar at all.
Soluble salts
Salts in the matrix migrating and crystallizing as humidity cycles, pushing surfaces apart. Stability matters more than any one humidity figure, which is an argument against a display shelf by a window or in a bathroom.
Acidic vapors from the storage materials
Some woods, boards, adhesives and foams off-gas, and in a sealed jar there is nowhere for that to go. Inert padding and an inert mount matter more inside a sealed container than outside one.
Abrasion, which is handling
A loose specimen is sanded by its own jar every time it moves. Mounting it so it cannot shift removes this entirely and it is the cheapest intervention on this page.
Light, for some material
Pigmented matrix and some mineralization can fade. The shelf offers 25 amber, 11 black and 3 frosted containers against 66 clear, so the choice between showing a specimen and protecting it is available.
Clear jars make all of this visible
Which is an advantage, not a problem. A bloom or a crack in a clear jar is noticed; the same change in an opaque box is found years later. For an active collection that visibility is worth a lot.
Inspect on a schedule, and write the date
Twice a year, checking for bloom, cracking, salt and desiccant condition. Everything on this page is slow, and only a schedule catches a slow failure in time.