The closure of big distributors such as restaurants means that big orders get cancelled. These cancelations cause a glut at the point of production.
However, the article fails to mention the role that agricultural subsidies play in the oversupply. Regarding dairy in particular, consider:
> "... in 2015, the support granted to U.S dairy producers represented approximately C$35.02/hectolitre - the equivalent of 73% of the farmers' marketplace revenue. USDA data also reveals that US dairy farmers operate at a loss, and have a cost of production that is higher than what they earn from the marketplace."
https://markets.businessinsider.com/news/stocks/american-dai...
This has been happening for decades, leading to artificially suppressed food prices and market distortions across many categories.