Stock prices first began to decline in late 1929 because

Investors began selling of their shares.

Explanation:

When investors sell shares in a company because they don't think it will sell for more in the future, the price of the stock falls. Other people will be less likely to buy the stock if they see the value falling, not rising, so the price will continue to drop in a dangerous cycle until something changes.

eds? ederal government or feds