I was wondering why myself.

This is what I found: Rail cars are not being unloaded fast enough (labor shortage) that UP says they have to limit traffic in order to clear the lines from stagnant railcars and will begin metering railcars to speed up the process.

https://www.bloomberg.com/news/articles/2022-04-12/union-pacific-will-limit-traffic-to-clear-clogged-rail-lines

edit: Looks like Krako beat me to it. I'll leave this here anyway.

This article goes into it in more detail https://www.fox5vegas.com/2022/04/15/fertilizer-company-complains-about-railroad-shipment-limits/ A little more complicated then Mr "Durden" would have us believe. Also I work in the produce industry and this daily news letter has a lot of good insights into the industry https://theproducenews.com/ Big story right now are the trucking delays at Texas border from Abbots stupid stunt. He reversed the decision that was causing delays but the damage to supply chain was already done and will take months to catch up.