Depending on the age of the helmet, I'm not surprised at all that it shattered. New helmets use new technology as far as plastic is concerned; often akin to hard rubber than brittle plastic. Older (brittle) plastics aged differently and the older they were the more they shattered when they reached a particular age. Also, elemental exposure that the helmet experienced in its earlier use, when it wasn't "ancient", would play a major factor in the degradation, and continued "brittling" of the plastic, especially if it was an early plastic helmet.

I use and recommend Pro Tec and Giro to those than can wear them.