When the stock market crashed in 1929, it was the end of a long decade of building momentum. The market crashed, people lost everything and banks were not re-paid their loans. The agriculture industry was hit especially hard.
Between 1920 - 1929 Americans demanded a lot of food and the farmers planted and grew amounts that would meet the demand. The high demand increases prices. When the depression hit, people stopped buying food (at least fresh food) the farmers could not sell their crops, prices dropped, and farms failed.
Banks loaned out money to farms and businesses, prices dropped, farms and businesses closed, and banks lost their loans. Banks closed - called Bank failure.
No banks means no money in the economy, no farms because of low demand, no manufacturing = Depression.
By 1932, unemployment was almost 30%. The Depression was about Jobs, Jobs, Jobs.