1. Exposure to ultra violet radiation (sunlight) and heat in storage.
2. Expulsion of stored gases in the expanded polystyrene material that is designed to absorb the energy of an impact by deforming upon impact. E.P.S. (Expanded Poly Styrene) is the "styrofoam" like material used in bicycle helmets. The helmet is then usually covered with an injection molded polystyrene (not expanded with gas, just regular hard plastic) shell. Increasingly some lightweight, high performance helmets such as the Giro Ionos used an intergrated, molded-in skeletal structure of polystyrene for increased structural integrity. These interior components do not assist in energy absorption during impacts. E.P.S. is basically plastic infused with gases to "puff it up" forming gas voids that compress upon impact. The gas leeches out of the polystyrene over time, reducing the helmet's ability to absorb impact and making it less protective- hence the three year replacement necessity with some models and brands.
3. Overall wear of suspension components such as chin straps, buckles, sizing pads and adjustable hat band sizing components. These things just wear out, especially with frequent use, adjustment, buckling and unbuckling and perspiration residue.

Bottom line: Replacing your helmet in compliance with manufacturer's recommendations is not marketing or some veiled attempt at selling you a new helmet every three years. It is good common sense based on independent engineering tests performed by scientific testing agencies not involved in the helmet sales industry. Generally, they are the ones imposing the recommendations for replacement to reduce liability insurance costs for manufacturers- it isn't the manufactuer necessarily making the recommendations.