Part of it originates with bait and switch tactics for Christmas shopping. For example, a chain store in a large newspaper spread in large bold type offers XBox360s for $100. In the fine print, it would be for the first 10 customers only, and after that it is the regular $150 sale price. Well for some people, they'll camp the night before and fight everyone when the door opens just to save $50 on an XBox. A sensible person would preserve their dignity and just pay the extra money. But there are plenty of shoppers who gladly play this game and truth be told they are probably enjoying it. So, whatever floats your boat. Welcome to America.
So that is why people fight each other in the department store--that is where that originated. Anyway, the media and marketers caught wind of this and started playing it up to the point where Black Friday is some kind of supplemental holiday, and people being the sheep they are do what their overlords command.