I think it was mentioned that he had in the order of 25% exposure to the Bank, shares and CFD's, so not strictly all CFDs. As BB mentioned, due to the size of the position the broker likely purchased shares in the market to match SQ's bet. So SQ didn't hold the shares directly, but they were effectively removed from the float. In the Anglo situation, the share price was falling. The broker would be holding the shares, however, the broker would also make the shares available for short selling, exacerbating the fall in the share price by lowering the cost to short the stock. SQ's position was underwater, so for the bank, they recognized that there was a high chance that SQ would close his positions, with the broker consequently dumping the shares they were holding, causing a further significant drop in the Share price. This what the market also suspected so traders kept up the pressure on the stock price.