Why happens as the Joads leave the camp?

After the Joads leave Hooverville they come upon a nearby town. An angry mob of men guard the entrance to the town. They have placards and shotguns. They want to turn away "Oakies" from entering their town. Tom takes exception to this but Ma Joad tells him that now is not the time to make trouble. She reminds him that a, "different time’s comin".