Why do currency crises arise and how could they be avoided?

In the 1990s currency crises arose in different regions, e.g. in Mexico, East Asia, Russia, Brazil and Ecuador, to mention only the most important ones. What are the main factors which may trigger such events? How does globalisation and deregulation of financial markets influence the emergence of a currency crisis? What forms of crises exist? Are they driven by fundamental imbalances in a country or are they caused by self-fulfilling mechanisms involving herd behaviour and destabilising speculation? To what extent do such crises reflect implicit governmental and international guarantees which may cause moral hazard and adverse selection, thereby increasing the risk behaviour of enterprises and banks?