Property values plummeted, thousands went bankrupt. Stock market scams were common on Wall Street. Insiders made huge profits. Buyers formed stock pools to artificially drive up the price of a stock, than unload it for a profit when it reached an artificially high price. 1928 was a banner year for the market.
Nobody knows what caused the wave of speculation.
In the 1920s an obsession with quick riches infected American culture. The market succumbed to greed in a decade when avarice was a virtue. Inexperienced investors and innovative financial devices fed the boom. Stockbrokers aggressively sold stocks. Money to fuel the speculation was plentiful because of the booming economy.