The Great Depression started out a lot like our recent Economic Crash. What made the whole situation much worse was there was a tremendous drought over the Plains states starting just after Black Tuesday. All the farmers in the midwest got in a great corn crop in 1930 (which tanked the corn prices and sent a lot of farmers bankrupt), then no one got any crops at all for several years. There was no rain.
That is what started the migration of farmers to the west coast, in the hope of finding field work. They were literally starving to death in the midwest.
Then, in 1935, they had Black Sunday - 14 April 1935. Unbelievably huge walls of dust rolled over the plains. So even though you may think that the folks from Oklahoma (in the book) were escaping from the Dust Bowl, that's not actually the case. The book starts in 1930. The actual "dust bowl" events didn't start happening till about 1935.
As tight as the job market is during the book, what Esperanza and her family don't know is it was going to get much worse, as more and more refugees from the midwest drought and the Dust Bowl descend on the relative prosperity of the California growing region, just hoping to survive.