Strategists at Citigroup believe the sell-off in the U.S. stock market last week makes it likely that major stock indexes will fall further. Strategist Chris Montagu and others said in a report to clients on Monday that the massive unwinding of long positions in the SP 500 index coupled with an increase in short positions in the Nasdaq 100 index suggests a shift in risk appetite towards a more outright bearish bias. Hedge funds' de-risking of the SP 500 has left total exposure at half its mid-July peak, they said.