How does buying on the margin contribute to the stock market crash?

In short, a person is buying with money they do not have, but are borrowing from their broker. A person does that so as to buy more of a stock they believe will go up in value. A broker does this because he assumes that the person has more information on this than he does, and wants to make money, too.

However, not all stocks go up in value, some decline. And then the person and broker are compelled to sell what stocks they have to cover the losses. If stock prices were already going down across the board, the dumping of another amount of stocks on to the market exacerbates this downward trend.

The above answer is very short, very simplistic and very generalized - I know, I'm the one who just wrote it. It will do for general purposes. For greater detail, google "margin" or consult a competent financial analyst in your area. So long as you are simply asking for information purposes, there are probably some college economic professors or local bankers who'd be more than happy to assist.