By late 2006 the rise in US interest rates had induced first a loss of momentum and then a turnaround in house prices. This triggered a domino effect, starting with the structured products based on subprime mortgages.
There's a pipeline going from one account to the other which is not instantaneous, and the bank receiving the money will often not let the recipient draw on it until it is sure that it has itself received the money from the paying bank.