This paper examines the microfoundation of macroeconomic tail risk, defined as the frequency of extremely negative GDP downturns relative to what is predicted by a normal distribution. We demonstrate that firm-level productivity shocks, even when normally distributed, can be amplified into large macroeconomic tail events through a dynamic production-based input-output network, challenging the traditional view that only sufficiently large firm-level shocks impact the aggregate market. The evolving network structure, driven by technological shocks, hinders risk diversification across firms, delays the mean reversion of aggregate volatility, and causes volatility to cluster from local to global scales over time. In bad times, when idiosyncratic risks are high and firms treat all inputs as complementary, these risk spillovers can accumulate, resulting in severe GDP downturns.