The plastic is absorbing humidity from the air during a long storage period to ambient air. Humidity on filament can change it’s properties (filament diameter, melting point etc…) and so cause issues on future prints. More over materials such as Nylon easily absorb humidity. It is recommended to store your materials spools in a dry area, under a sealed plastic with some dryer bags. You can also pre-heat the spools by placing them on your heat bed for 10 to 30 minutes. This technique is even more recommended for materials such as Nylon.