What was a cause of decreased demand for food in the 1920s?

Overproduction with farmers

According some author the cause was the decline of agriculture.

Explanation:

The decline of agriculture in 1920 had different causes as:

1) Overproduction. It was linked with by several others factors like overly efficient machines.

2) Governments polices. US in that time was trying to sell its abundance of goods to foreign countries.

3) New Machinery. The new technology that was invented such as tractors produced that farmers losing their jobs.

4) The effects of World War I. It was not directly but after an efficient context it was start to decline.