It's the snowball effect. And a self fulfilling prophecy. Knuckleheads believe there will be a shortage, and start buying up big. Slightly more intelligent people start to worry the knuckleheads are right, and join in. Average people buy a couple of weeks worth, and the shelves start to look barish. Other average people say, I can see where this going, and buy what they can. The knuckleheads sit back smugly, and say See! I told you there'd be a shortage!
A self fulfilling prophecy, driven by knuckleheads and dolts!