What caused the stock market boom of the 1920s?

It was credit people became interested in credit

The economy began to contract and professional investors started selling their stocks and share prices began to slowly fall. Stockbrokers began to make large-scale margin calls, demanding that investors repaid their loans all at once. Panic started, smaller investors, worried about paying off their loans, began to sell and stock prices fell even further. The stock market went into free fall and on Tuesday, October 29, 1929 (nicknamed Black Tuesday) stock prices completely collapsed. Between $10-$15 billion was lost in just one day due to the plummeting share prices and the 1929 Wall Street Crash.