Because they’re hungry. See this very good analysis from Richard Spencer.
As David Hacket Fischer demonstrated in his monumental work The Great Wave (1996), commodity-price spikes—and related governmental interventions—regularly coincide with political violence and “regime change.” The 18th century, for instance, was an era of higher prices and political revolutions, most prominently in America (1776), France (1789), Switzerland (1792), Belgium (1794), the Netherlands (1794), Poland (1794), and Ireland (1798). In the French example, the Bastille Day riot (14 July, 1789) coincided almost exactly with a cyclical peak in grain prices. In turn, Robespierre fell from power when a public riot ensued after he had instituted wage controls. The whole era of instability in France was inaugurated by John Law’s infamous “Mississippi Bubble” inflation of 1719-20, which led to the destruction of the market for royal billets d’état and a near total economic collapse.