It will happen this way:
One US morning, interest rates in the Euro-Zone will have crashed the "Euro" enough to create a small but significant sell off in order to "cover" the new underlying (junk sovereign bond) loss for some intermediate US "laundry-house" bank or shadow-bank.
Once that "bank" sharply sells off its risky Euro-Zone assets, some other similar banks will be forced to sell off sharply because they have invested in the initial "shadow-bank-to-fail."
Then, all US banks (except community banks) will sell off the assets they have on deposit with other institutions that have all invested in the shadow-bank program.
And then... everything has to sell off. Just as in 2008. Again.
The fall in US markets so far is independent of the debacle to come.