Answer: Stocks became overvalued.
Detail/context:
There was much speculative buying on the stock market in "the Roaring '20s," as the decade was known. In the 1920s, people were so eager to invest and earn profits through the stock market that they bought stocks "on margin." In other words, they paid for only a marginal percentage of the stocks with their own funds, and borrowed bank funds for the rest of the purchase. That meant the banks were complicit in this arrangement too, by allowing those sorts of loans. By the late 1920s, 90% of the purchase price of stocks was being made with borrowed money. This inflated the market in a way that spiraled out of control, with stock values growing much higher than was in line with reality. In 1929, the market crashed. This was the event that initiated the Great Depression that took hold during the 1930s.