These were rare before the mini-crash of March 1929. But when they did come, in the mini-crash and in the Stock Market Crash of October 1929, margin calls ensured that more stock would be sold as brokers tried to make back some of what their clients owed them. The volume of stock sold in October 1929 created the Crash.

With the decrease of exports to Europe (rebuilt, European nations have drastically reduced imports of industrialized and agricultural products from the United States.), the North American industries started to increase the stocks of products, because they were no longer able to sell as before. Most of these companies had shared on the New York Stock Exchange and millions of Americans had investments in these shares. In a nutshell, producing a lot, selling a less.

Explanation:

In October 1929, noticing the devaluation of the shares of many companies, there was a rush of investors who wanted to sell their shares. The effect was devastating, as the shares depreciated sharply in a few days. Very rich people passed, overnight, to the poor class. The number of company bankruptcies was huge and unemployment reached almost 30% of workers. The economic crash, also known as "The Great Depression", was the biggest in the entire history of the United States. As at this time, several countries in the world maintained commercial relations with the USA, the crisis ended up spreading to almost all continents.