The way you phrased this question has a fundamental problem: It assumes the a major historical event becomes a major historical event solely because of events before the event begins. An event does not play out the way it does because solely, or even partially, due to what caused the event.

Every historian will say that the reaction to the Wall Street Crash of 1929 is just as important, if not more important, to understanding why the Great Depression was so long and devastating as it was. And they would agree on that even if they disagree on what should have been done.

The crash of 1929 was large, but other crashes before and since have been larger. It was the reaction to Black Thursday that was different. This is the reason most scholarship on the Great Depression is focused more on the Hoover and Roosevelt administrations' actions than the Coolidge administration's actions.

While we can look at the Coolidge administration and ascribe to it's policies much of how the American economy had gotten to where it was in October 1929, the reaction to the crash was wholly the responsibility of the Hoover and, later, the Roosevelt administrations.

That is not to say that the economic environment that had been created in the Coolidge years did not contribute to the Stock Market Crash of October 1929, which was the immediate cause of the Great Depression.

The American economy had grown tremendously during the Coolidge years. One of the results of this growing economy was that many people had begun to buy stock in the Stock Market. Some had even borrowed money to buy stock. And a lot of those stock purchases were mostly speculative in nature, people had hoped to buy a stock that was going up and sell it for a profit based on hype rather than actually investing. The people who had borrowed money to buy those stocks were essentially gambling and hoping to score it big to pay back their loans and make a profit. South Florida was rife with land speculation schemes that cost people their life savings. The Marx Brothers made a comedy about it called "The Coconuts" based off of Groucho Marx's own tremendous financial losses investing in Florida. Once enough of these, essentially, con artist schemes went bad a lot of people owned worthless stock in defunct companies and owed tons of money to banks.

That is the classic problem everyone blames for the Stock Market crash. Speculation and debt.

But there were other contributing, less romantic factors.

In early 1928 the Fed increased interest rates. Soon after the economy began to slow and by August of 1929 the American economy was in recession. Who is to blame for that is up for debate.

Now on to that bad weather that was ruining crops in a still agricultural American South and Midwest. Crop failures left farmers unable to pay back banks, unable to borrow more (which is key to the business side of farming), and left their banks bankrupt.

Once banks started going under, the entire financial system was put under strain. But after the crash of October 1929, things got worse. How people should have responded is up for debate, but most people will blame how great the Great Depression became on either the Federal government for not doing enough or on the Federal government doing too much once it began.