For months now, commentators and economists have been warily eyeing the US economy and asking what will cause the next recession.

Risky corporate debt seemed to be building up in the system, but with profits robust and interest rates low, servicing the debt didn't seem to be a problem. The Federal Reserve raised rates by a modest amount, but signalled that it did not have much appetite for future increases. President Donald Trump's trade war didn't seem to be having much effect yet. Then came the government shutdown.