This article suggests two reasons for dividing multi-tract/multi-owner farms: allows landowners to make separate decisions on the new farm programs (ARC & PLC) and allows operators to put some farms in one program and some in the other, hedging their bets.
Maybe we have a sort of Platonic ideal of what constitutes a farm, perhaps based on Jefferson's image of a nation of yeoman farmers? If we do, our reality doesn't match the ideal. When I joined ASCS it took a while to figure out that agriculture in some areas, the north and west, differed from that in the South, specifically in the balance between landownership and farm operation. Big landowners in the South, who often called the shots for the producers; small landowners in much of the rest of country, renting to operators who farmed multiple ownership tracts.
As the emphasis in ASCS/FSA programs changed over the years, the incentive for operators to combine or divide changed. Production adjustment programs meant combinations so you could designate the poorest land to be taken out of production. Disaster programs meant divisions so a loss on one tract wouldn't be offset by production on other tracts.