Why did the stock prices drop so quickly in 1929?

By then, production had already declined and unemployment had risen, leaving stocks in great excess of their real value. Among the other causes of the stock market crash of 1929 were low wages, the proliferation of debt, a struggling agricultural sector and an excess of large bank loans that could not be liquidated.

Explanation:

There was much speculative buying on the stock market in "the Roaring '20s," as the decade was known. This inflated the market in a way that spiraled out of control, and in 1929 the market crashed.