Several factors could be at work... Boiled linseed oil mayhave been used without proper driers... The never-cured oil may be coming to the surface. There will probably be a light, sour odor if that is the case.
More likely, solvents in the closed safe have softened the oil. Putting the stock in a hot area for a day or two should do much to firm it back up. A coat or two of Tru-Oil will prevent it from happening again.
If heat does not cure it a wipe with Formbys Furniture refinisher will take a little off the top and speed things up. The heat will reduce relative humidity, a complicator when oils get soft...
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