The problem for investors in the stock market crash of 1929 was something called overnight call loans whereby the lender could call the loan in with 24 hours hours notice.
Many morans were suckered into taking out these loans not suspecting that the lenders would deliberately call in the loans at the sametime.
Because the stocks were bought with loans, they did not have the cash to repay the loans at such short notice and everybody tried to sell their shares to raise the funds. Naturally, because everyone was doing it, the price of the stocks crashed.
https://people.brandeis.edu/~cecchett/Polpdf/Polp05.pdf