Some of the expansion depended upon the booming stock market where prices were inflated by rampant buying on margin for stocks people considered attractive. (A person who buys on margin puts up only a fraction of the price of the stock and borrows the rest. Fine as long as the market keeps going up.) When stocks declined, many of these borrowers couldn't pay what they owed. They went bankrupt. That in turn caused further sell offs. The market collapsed. Thus the crash.
Mass production and production lines gave many people jobs, drawing worked from rural areas to better paying jobs in the city. However, there were social upheavals and poor conditions for factory workers. Some of these conditions seemed to fulfill Marx's predictions in Das Kapital, making the country ripe for a social revolution. When the stock market crashed and businesses went belly up the workers were left stranded, with no jobs. Franklin Roosevelt tried to stave that off and did some good through government created jobs, but only World War II really fixed that situation..