The ultimate source of this long-run tradeoff is the existence of shocks to aggregate supply. Consider the canonical example of an aggregate supply shock, a sharp rise in oil prices caused by disruptions to foreign sources of supply. According to conventional analysis, an increase in the price of oil raises the overall price level (a temporary burst in inflation) while depressing output and employment.
Monetary policymakers are therefore faced with a difficult choice. If they choose to tighten policy (raise the short-term interest rate) in order to offset the effects of the oil price shock on the general price level, they may well succeed - but only at the cost of making the decline in output more severe.