when volatility is low the firm can borrow and buy more on margin because the loss potential in dollars is low.
but when volatility picks up they have to unload and sell or risk margin calls and losses bigger than they want . so all these computerized programs move together and selling leads to more selling .
the economy world wide is growing , unemployment is low , rates are still historically low and we have 81% of companies earnings being beat . throw in the tax cuts and this will hopefully be a flash in the pan because nothing changed . i bought today .