One theory about why consumers are spending less on agricultural goods is that goods are becoming cheaper to make and manufacture, so consumers are receiving the savings. Develop an alternate theory as to why consumers are spending a smaller percentage of their incomes on agricultural-based goods.

Another theory about  why consumers are spending less on agricultural goods is:

Thanks to the free trade agreements between the US and other countries with a more centralized production in specific agricultural products, for which said products enter at a cost below the cost of national production, consequently, a lower cost for the consumer.

Explanation:

There are countries which agricultural goods production is more focused in a product, for example, suppose that the USA has a free trade agreement with Argentina, where the potatoes production is completely focalized, the reason why his production is cheaper than in the USA (by a economy of scale), then, when that good arrive to the country, will be cheaper than the national production and the consumers will pay less than before.