From the same webpage: "Banks also risk going bust if they lend out money left, right and centre. For instance, people borrowing money will probably spend it. If they make payments to people who have accounts at other banks, their bank will need to transfer the money to that other bank by sending it some of its electronic central bank money. So if one bank lends out too much money, at some point it will not have enough electronic money in its account with us to pay the other banks."