Answer:
Many people's wealth is held in stocks and as the price of stocks collapsed, they lost wealth.
Imagine that this happened to you. One day you are rich and that affects your spending habits. In a matter of few days or weeks, you lose a large portion of your wealth. So now, you are less rich or even poor. So your spending habits will be altered, i.e. you will spend less.
If you consider the economy as a whole, aggregate demand will fall, resulting in a decrease of aggregate supply, and an overall decrease of the GDP.