The risk of deflation. Energy prices are a big component of the CPI. As they fall, so does the CPI. Take for example for 2016 there was no COLA for Social Security. So seniors had less money to spend. So while the $20-50 per month for an individual is not much, collectively it is a lot. Then as a society, once we think things are going south, we quit spending further sending us into recession territory. And this happens because information is not perfect. To those in my state with coal prices falling, even those who don't directly or indirectly rely on coal for a living might be tempted to save a bit more and spend less even though they are not directly affected. Then it all just tumbles downhill from there.