The first cause was an unsustainable increase in credit, creating a self-reinforcing boom/ escalation of share prices. As shares were used as collateral and bank reserves, higher share prices permitted more lending which permitted more purchase of shares leading to higher share prices...
The Federal Reserve bank belatedly responded to this bubble by tightening credit. This caused overall credit to contract as less credit forced share sales which lowered share prices, leading to less collateral and reserves for lending which conditioned more share sales.
In addition, many countries responded by raising tariffs, such as the Smoot-Hawley Act which suffocated trade and retarded the development of more competitive businesses. Otherwise, governments were reluctant to directly interfere or take actions that might counteract the "natural" behavior of the markets.
In the U.S., unsustainable agriculture practices combined with adverse weather to ruin harvests. In turn, farm failures drove banks that lent to farmers to also fail. As these farmers' banks failed, they in turn defaulted on their loans to other banks, aggravating the problems mentioned above.
The global impacts were a loss of trade, contraction of output and prices, and an increase in unemployment and forced migration.