Maybe it's because the US is now a massive oil producer, but there are a lot of oil companies in a lot of debt, so depressed oil prices will force them to go into bankruptcy. Next step will be US government propping up the industry somehow, IMO, either with a bailout or by buying oil at above market prices and increasing strategic reserves.
Is it oil prices down -> large oil based economies at risk due to vastly lower margins -> global instability from inability to make payments?