Some folks like gambling
But the odds are getting better for a few knocks on the market. China is obviously one and another is internal domestic economic problems.
Another is a decision by OPEC or a major oil producing country to change oil trading currency. This is already on the table being discussed, but no rush. However if the dollar continues to lose value - the likelyhood of a change in petro-currency rises. The effect of that if it was agreed by all OPEC countries, would be total disaster for US markets - and far supercede any effects suffered after 9/11.
Sure, a terrorist attack can and will happen any time, but the odds of turmoil in the market is slowly increasing at various levels without the "help" of any attack scenarios.
To gamble any of these elements may happen before late Septemeber is cutting it fine *lol*