US farmers had been overproducing crops since world war 1.
The United States had to supply its European allies with food.
Using machinery greatly improved farm technology and then tractors started being used.
Most European nations had a lot of post war problems. So many countries had to rely on the US for food but they had no money to pay for it.
Farmers were still selling more crops than they could sell which made the price for farm products fall drastically. President Calvin Colidge did nothing about this problem and said that farmers never made money in the first place.
Efforts in congress failed and couldn't protect US farmers from foreign competition. This put them in debt so the farmers took loans from banks which made banks go bankrupt mostly rural banks.