RotJ yellow bubbles

Plastic is organic in origin, ie derived from oils. Anything organic is going to degrade and decompose over time. Plastic bags might be the scourge of the environment but ultimately plastic isn't an inert substance so heat, light and humidity all play a part in the breakdown of the material at molecular level over a period of time. Plasticisers leach out and the material becomes brittle and discoloured. Heat will speed up a chemical reaction so stands to reason those figures that have been stored in hot humid environments will go first.