Correlated shifts in expectations are the key to understanding what happened. The fundamentals (such as when a thing gets cheaper, people buy more of it) were overwhelmed by something akin to panic - if everyone is running in one direction, we should run too (because it becomes increasingly costly not to).
Lower trend growth  As to the second trend, evidence for lower trend growth can be derived from Okun’s Law, which describes the empirical relationship between unemployment and the decline in a country’s output. In the next graph, annualized quarterly changes in the unemployment rate and annualized quarterly changes in growth for the United States are plotted on the x and y axes respectively.