And, as Nanex points out, S&P 500 futures liquidity is collapsing today.

Why? Because whereas the BTFDers have been willing to jump in and, well, BTFD, on days where there is a sharp move lower, both the HFTs and the carbon-based traders step aside and pull their bids, unsure if this is "the start" of the selloff.  Maybe this time they are right, as the bank bloodbath continues: