The answer is:
Stockbrokers who still had profits on their books were afraid that their profits would disappear.
Stockbrokers who had losses were afraid that those losses might get larger.
Investors decided to get out of the market.
The stock market closed on weekends. So, when these overwhelming fear build up among investors over the weekend, the majority of investors in the market decided to quickly get out of the market by the time it reach monday. This make the capital in the market taken out too rapidly and contribute to the colapse.