The answer is: Agriculture
During the gilded age era, a lot of people who live in rural area decided to move into urban area to seek job opportunities in manufacturing industries.
Due to the industrial revolution, the migration does not negatively affect the total agricultural production since the labors can be replaced with machines. But, despite high production levels, the agricultural companies in rural area do not have enough customers to buy their products since a lot of them already relocated to the cities.
This cause a massive overproduction in the agricultural sector.