They get brittle over time. I had some fly apart too. Don't stock up. Manufacturer's suggest a 1 year lifespan. They'll last longer than that but don't push it. They say that the sandpaper sheets also deteriorate. I just bought a bunch from an outfit named Keen here in Canada and they say store them in a ziplock bag.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.