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 positive news, which, due to a feedback mechanism of these news on the agents' strategies into collective transient herding regimes.