What circumstances bring the wealthy class and the poor class into conflict with each other in The Grapes of Wrath?
I would say that the scarcity of money is what brings the rich and poor into collision with one another in Steinbeck's work. The poor consistently interact with the rich out of anger in the disproportionate nature of wealth that exists between them. In the narrative, there is a strict demarcation between rich and poor and the only time when their worlds collide is over wealth, with the rich protecting their interests and the poor seeking a share of theirs. Given the time period's lack of wealth for anyone, this makes competition particularly brutal and one where antagonism marks each encounter between both groups.
I actually think that the land turtle in chapter 3 might be indicative of the struggle between rich and poor. The poor in Steinbeck's work seek to continue on their way, struggling through adversity and avoiding contact with swerving vehicles, in this case representative of the wealthy. The endless antagonism over wealth eventually causes one or two cars to hit the turtle and force it on its shell. This is representative of the poor's struggle to battle through its formidable challenges with the rich. As Steinbeck features the turtle righting itself and continuing on, so too do the poor find a way to cope and continue on their own path for happiness. The symbol of the turtle is representative of the circumstances and conditions that bring the wealth and poor class into conflict with one another.
This metaphor is repeated in different settings in the book. The "bankers" come to represent one aspect of wealth pressuring the poor, like the car that swerves to knock the turtle down. The poor looking for work, only to find signs that say, "No Help Wanted. No Trespassing" is another example of how the antagonism between rich and poor exists, or how the land turtle is knocked down by conditions and circumstances. Finally, the bosses who seek to exploit the migrants by offering them "dirt" wages and undermining their efforts again display the dynamic of the turtle getting knocked over and having to right itself.