Thee cause was probably a combination of speculation, leverage, overbuilding, and so on.

No one really knows the exact trigger, but a popular opinion seems to be that the Smoot–Hawley Tariff Act made it worse -although even that is debated.

The stock market crash lead to margin calls and the sudden evaporation of millions of dollars of wealth and that probably triggered a cascade effect, as business obligations could not be met due to the sudden loss of wealth.