Ingrediants to a nasty selloff:

(1) Longs have no more or limited money to invest (re: funds outflows and insider sales)

(2) Longs will turn and sell this market quickly to protect/book gains made in the last 3-6 months

(3) Few shorts are left to bid this market up

(4) New shorts will enter (reinitiate) the market on weakness

(5) Government money is getting tapped out (politically, and with the public, debt ceiling)

(6) Underlying market fundamentals are weak

(7) Seasonals, cycles, and technicals strongly argue for a top and selloff, and

(8) Computer led programs and quants will pile into a negative momentum led selling