It was bad engineering. The impeller (circular finned disk that pushes the water through) is made of plastic. It can become weak, develop hairline cracks, then eventually splits apart into a couple or sometimes a million pieces.
The replacement ones by GMB (I think that's the company) are metal.
This was the how-to for future reference
http://www.contour.org/mods/mods.cgi?s=howto&display=waterpump 
