Radiation dose

Temperature history

Ozone exposure (or other oxidizing agents)

Mechanical loading (part may sag, deform significantly over time)

UV exposure (or other wavelengths that degrade compounds)

etc.

Just to add, the particle size and surface area are bulk (physical) properties that may also play a role particularly in heat transfer during thermal conversion. In addition, the proximate (volatiles, fixed carbon and ash) are also important material properties. Hope this helps.

BBN

Temperature, exposure to air or relative humidity also affect the properties. These consequently tend to affect the shelve life.