In addition to what you know of today's action, which was driven by Europe, the Markit CMBX index, which is 100% U.S.-based, is crashing. LINK
For whatever reason, I have followed AA.4 the most closely. It has now dropped below the November 2011 low. One reason I follow this index is that it has been given to long intermediate trends. Thus to date it has had predictive value. This then brings me to thinking that continues to be bearish. Leaving aside the trading strategies of JPM and its London Whale, and other int'l financial groups, the U.S.-based banks ranging from certain microcaps and much larger co's I follow such as NTRS and UMBF are at or near multi-month/multi-year highs. Yet their credit quality should correlate with CRE credit quality, of which the CMBX indices are one way to view that metric. (Not the only one...)
If the ongoing drop in Treasury yields were due to Fed action with the strengthening economy that so many foresee here, then I'd expect to see European money flee Europe and bid our shortest-term rates down. Instead they are holding steady, thus the spread between the 10-year and the short rates has been narrowing--another recessionary sign, and entirely consistent with the message I take from the CMBX indices.
That is that there is growing empirical evidence to support the "recession 2012" view.
Even a mild recession could be associated with a disproportionate drop in stocks relative to drop in economic activity.