That's because Trump is flooding them with federal monies in the months preceding the election. According to an article in yesterday's New York Times, "Federal payments to farmers are projected to hit a record $46 billion this year as the White House funnels money to Mr. Trump's rural base in the South and Midwest ahead of Election Day."
But make no mistake about it: If Joe Biden were president, he would be doing the same thing, probably to an even greater extent, maybe to people other than farmers. Liberals (i,e., progressives) have long been infamously famous for using the state to take money from those to whom it belongs and give it to those to whom it does not belong. They call this system of mandatory charity, which is based on IRS returns, audits, prosecutions, and incarcerations, "care and compassion."
This is what the welfare-state way of life has done to Americans. It has made many of them dependent on government largess that is taken from others. It has also made them eager to support any candidate who promises them the most money and other benefits.