Before revision:  We find that rallies and crashes occur as amplifications of random lucky or unlucky streak of news, due to the feedback of  these news on the agents' strategies into collective transient herding regimes.
After revision:  We find that rallies and crashes occur as amplifications of random lucky or unlucky streak of news, due to the feedback of these news on the agents' strategies develop into a transient collective herding regime. After this self-amplified exuberance, the price has reached an unsustainable high value, being corrected by a crash, which brings the price even below its fundamental value.