Why did stock prices decline in the late 1929?

They went down because everyone wanted their money when the banks were very poor.

Answer:

Many people, including many in the general public were buying stocks on "margin". Everyone thought the stock market was a sure bet to make money. If the stocks value went down the people had to come up with more cash or sell the stock. Others tried to benefit by pushing the stocks lower. Since many people had large parts of savings in the Stock Market many lost their savings. Many businesses (including banks) had their excess cash invested in the market. When it "crashed", many people lost everything, banks didn't have money to cover all of their deposits and failed, businesses lost their operating capital, their customers lost their money and everything came tumbling down.