Boom-bust cycles are a prevalent feature of economic fluctuations, but there is no consensus on how they come about. Are these cycles the result of recurring positive and negative shocks, or do they arise because of processes internal to the system Empirically, we find that positive shifts in expectations generate boom-bust dynamics, whereas technology improvements do not. We rationalize our findings in a Real Business Cycle model with an endogenous borrowing limit. In the model, credit market amplification plays a central role during expectation-driven expansions, while it barely shapes economic dynamics after technology improvements. Consistent with data, both types of expansions are characterized by an increase in credit growth, suggesting that policies aimed at limiting all credit expansions might not be optimal.