Why has Cairo unrest erupted again?

In scenes reminiscent of the mass demonstrations that brought about the downfall of former Egyptian President Hosni Mubarak in early 2011, thousands of protestors have turned out in Cairo's Tahrir Square over the past week.

Source: http://edition.cnn.com/2012/11/30/world/meast/egypt-unrest-explainer/index.html?eref=edition

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