Why did investors start selling their shares in October of 1929

By then, production had already declined and unemployment had risen, leaving stocks in great excess of their real value. Among the other causes of the stock market crash of 1929 were low wages, the proliferation of debt, a struggling agricultural sector and an excess of large bank loans that could not be liquidated.

the answer to your question is a because laws act as guidelines as to what is accepted in society. without it there would be conflicts between social groups and communities. it serves as a norm of conduct for citizens.