What is the cause of sharp drop in exchange markets?

Stocks dropped sharply on Asian and European exchanges, as investors worried that the pace of a global economic recovery is slowing. Shanghai’s stock market fell almost 6%. Japan’s Nikkei Stock Average lost more than 3%, while Hong Kong’s Hang Seng dropped over 3.5%. European markets seemed to be following the trend with declines in the 2% range in Britain, France and Germany.