What seems to be the connecting link between the Bank, the owners, the tractors, and the driver?
The original question had to be edited. One of the connections between the bank, the owners, the tractors, and their drivers is that they all are a part of the machinery that removes the farmers from their homes. The crushing nature of capitalism and what it does to the farmers is embodied by the banks, whose drive for profit convinces the owners to remove the farmers off of their land by sectioning off the land with the tractors. They all are a part of the system that seeks to crush the voice of the farmer who has fallen on incredibly hard times.
Part of this connection is how Steinbeck depicts a capitalist system in which no one is really happy. Everyone has their distinct part to play in the condition of capitalism and the result is that each cruel role dehumanizes the individual and ruins lives. Steinbeck describes such a condition in chapter 5:
Some of the owner men were kind because they hated what they had to do, and some of them were angry because they hated to be cruel, and some of them were cold because they had long ago found that one could not be an owner unless one were cold. And all of them were caught in something larger than themselves. Some of them hated the mathematics that drove them, and some were afraid, and some worshiped the mathematics because it provided a refuge from thought and from feeling.
The math of profit is what connects the bank, the owners, the tractors, and their drivers. Each of them are pat of the "mathematics that drove them." The need for profit is what connects everyone, like a monster that binds: "The bank—the monster has to have profits all the time. It can't wait. It'll die. No, taxes go on. When the monster stops growing, it dies. It can't stay one size." In the pursuit of profit, the connective thread that also binds the bank, the owners, the tractors and their drivers is that they all end up crushing the farmers. The farmers are crushed under the heel of this system. The tractors are instruments that plow through and "rape" the land. The drivers who drive them do so coldly, doing it for the "mathematics of profit." They are emotionless about what they do, taking this "refuge from thought and feeling" and only knowing that the bank "pays good." This idea is another location where the bank, the owners, the tractors, and their drivers are all connected to one another.