Low interest rates obscure the risk, but the risk does not disappear. Both professional investors and private investors, who lend to their securities accounts and have bought shares at a pump, have felt this bitterly. Because the stock markets collapse, the lending limits of the custody accounts fall, and brokers fear for their loans secured with shares. Investors are asked to add money (margin call). If they cannot, the shares will go on sale automatically. If this fate strikes many investors at the same time, this increases the pressure on the prices. That is exactly what has happened again.